Welcome to my comprehensive guide on using on-map artillery a.k.a the 'king of the battlefield', the most useful tool in game and IRL for both defensive and offensive anti-infantry duties. Knowing how to use your on-map artillery effectively can change the tide of a battle, allowing you to easily stop an attack of multiple infantry companies dead in its tracks.
I will be using a German mortar battery as my example in this guide.

A Single Mortar

Here I have selected a single mortar unit, note: I have turned on the LOS view (no.3)

• 1: The Fire-at-will order, it's on by default. If you turn this off the gun/unit will stop firing.
  
• 2: The AI fire control order, units will open fire based on targets given by other units within the chain of command (explained further in the guide).
  
• 3: The Line of sight view of that unit, what you see visually may differ with what the unit sees, so you can use this tool to check. Anywhere within the green visual area can be seen by the unit, and vice versa with the red area.
  
• 4: The Indirect fire arc, similar to LOS, green area is where the selected artillery piece can indirectly fire onto, imposed due to the limitations of physics and gravity. Important when placing arty to check this, otherwise they may be staring dangerously at the enemy.

On the LHS of the screen is the expanded detailed info of the selected unit, the key point is that the unit operates on voice comms and has over 150% in ammunition (note: this is on battle editor).

A Mortar Battery

There are 3 main components to a battery:

• The first is the Battery Commander (Nagel,green circle), the Battery Commander, is the boss and has overall control over all units within the battery.
  
  
• The second is the Wire Squad (telephone symbol, blue circle), it's task is to link all commanders within the battery onto wire comms. The wire squad will automatically move to establish comms between the Battery Commander and the fire Platoon Commanders as long as AI manoeuvre is turned on (default is set to on). Note: in this example there is currently no wire comms between the Battery Commander and the firing Platoon Commanders (you can check this on the LHS of the screen; expanded detailed info).
  
  
• The third is the Individual firing platoons (in red), there are 3 firing platoons in this battery, with 2 guns per platoon in this example, totalling 6 guns. A firing platoon is comprised of its own Platoon Commander (Elter, Heidemann, Gottwald) along with its respective guns. It is important to keep Platoon Commanders near their respective guns if you want to engage in manual fire missions. This is due to the order of battle and structure of command (Battalion > Company > Battery > Platoon) which for some reason only applies to manual fire as I found out in testing. You can ignore this if you do not care about manually ordering your own fire missions.
  Just worry about ensuring arty is placed within voice comms range of any commanders within the battery.
  
  A firing platoon with its respective Platoon Commander
  

This means that there are 2 ideal placement structures one should adhere to:
1. Placing under all guns within the battery to be under voice range of the Battery Commander and leaving the firing Platoon Commanders to be spotters.

2. Place the Battery Commander as the primary spotter and letting the Platoon Commanders be within voice range of their respective guns.

Example: Left: 1. Battery concentrated within Battery Commander voice range, Right: 2. Battery dispersed using Platoon Commanders. Note: the LHS of the screen regarding "Link: Established" and "Link:Lost", denoting wire comms status.
Notice the hill terrain feature in the example, preventing LOS North for the battery but not the spotter(s). If an enemy troop concentration was to attack now, only the Left example will fire as it has established wire comms. Right will not fire despite correct set up untill wire comms is established by the wire squad.

There are pros and cons to each placement:

1. Using Platoon Commanders as spotters: More spotters, if a spotter gets killed then the battery still has spotters to order AI indirect fire (provided wire comms is established). Note the Platoon Commanders cannot issue manual fire missions for the whole battery due to the order of battle. For beginners its best to chose this set up if you don't plan on ordering your own on-map arty missions; as spotters are typically more exposed, and more spotters is better for redundancies. However this set up is probably not the best option when you are not entrenched (no pre-established wire comms) as it can take a long time to set comms, time you may not have in an attack.

2. Using the Battery Commander as spotter: You can disperse your firing platoons within a wider area, preventing enemy arty from wiping out your whole battery in one go. It also allows for more intuitive and easier use of manual arty missions (provided wire comms) since the spotter is in charge. I personally juggle between the use of either set up depending on the situation.
A good tip here would be to place the Battery Commander with the best LOS position you have with his Fire-at-will turned off to prevent him from revealing his position.

Note: I will mention a lot about manual fire because its really useful for firing upon enemies not spotted, this is not possible with automatic AI fire designations and incorrect order of battle set up.

**** IMPORTANT****

Voice Comms Range

Voice comms is determined as 75m away from any given unit in this game, meaning that you should always place guns within 2 deployment squares from a commander, if a gun does not have comms with a commander, it cannot receive indirect fire support missions and is therefore left to fire given its own LOS.

Wire Comms

(The green arrows show comms flow; edit: I turned on an option within the top UI bar to see this; should be within the 3 line box if I remember correctly).

Usually, unless a company/battalion is entrenched (stays still on the operation map for 1 turn), commanders do not start with wire comms between themselves, meaning that within the tactical battle, you have to wait for the wire squad to establish comms. If a commander has no voice nor wire comms established to a spotter, and no LOS on targets, no fire missions will be conducted.

One way to get around this is also placing Platoon Commanders in positions of LOS, with their guns located within 2 tiles for them to conduct its own indirect fire via local voice comms. I recommend to keep guns out of direct LOS but with clear indirect fire arc to the enemy as these are precious tools that can stop a large infantry assault dead in its tracks, and you don't want to lose them to direct small arms fire unless you have to.

Alternatively (not tested but should work in practice), have one Platoon Commander as the spotter with clear LOS within voice range of the Battery Commander; whom is within voice range of all guns in the battery to conduct AI indirect fire missions.

Tip: Don't place spotter(s) too far away from your battery, as it can take a long time for your wire team to establish wire comms (you can place them far away if you are entrenched: wire comms come pre-established).

Tip:: Once have an established wire comms, if you MOVE the unit, say the spotter, the wire comms will be lost and would need to be re-established. So be aware of the static nature of spotters once they are in place, try to place them away from the expected direction of contact in deployment.

So basically, if your battery isn't firing its probably because they either don't have LOS, or more likely there is no wire comms between commanders.

Lets start with a simple set up:

Here we have the Battery Commander (Nagel) in a spotting position, ontop of a hill terrain feature (dashed line) with good LOS. The whole battery is located behind a hill with no LOS to the enemy, thus without the spotter, the battery will not fire as it has no visuals.

The Battery Commander (Nagel) will order fire missions upon spotting the Ivans, he will issue AI designation fire orders automatically (provided you leaveAI fire control on by default, and Fire-at-will on) to the fire Platoon commanders, who subsequently will pass the order to its guns to start firing.

Reminder: This only happens when you have wire comms established between spotter (Battery Commander) and the fire Platoon Commanders. If there is no wire comms you will have to wait for wire squad to establish one, which sometimes may not happen in combat due to them being pinned down or killed.

Another set up: A Platoon Commander as a spotter
Now the Battery Commander (Nagel) is located within voice range of all guns of the battery, out of direct LOS of the enemy. There is an established wire comms to one of the fire Platoon Commanders (Elter). Elter, acting as spotter, will call out targets spotted to its North, and request fire mission orders to Nagel, and the battery will subsequently open fire.

Another Set up: 3 Platoon Commanders as spotters
Same terrain feature as before, but this time with 3 Platoon Commanders as spotters; if one gets killed the others will still be able to call in fire support.

Another set up: A commander from another company (black circle)
Given that there is wire comms established within the chain of command, other commanders from other companies/infantry platoons can request fire support and act as spotters (although you cannot use the manual fire feature).

Tip: The 3 small white dots above a unit commander indicates this wire communication between different commanders.

A Key note on AI fire designation missions:

After testing, it can be found that order of battle does not matter for the automatic AI fire missions (but it does with manual fire!).
With the above set ups, the Battery Commander (Nagel) is interchangeable with another fire Platoon Commander from the battery (i.e Gottwald) as the commander within voice range of the guns. However this is assuming that all Platoon Commanders have established wire comms with the Battery Commander (Nagel). So Set up no.1: where the Battery Commander within voice range for the battery, is best if you want to have a smooth operation with minimal micro/use of manual fire, as it avoids any unwanted breakages in the chain of command since the Battery Commander is not exposed more exposed to the enemy (if Nagel is suddenly KIA, another Platoon Commander cannot issue orders for the whole battery due to rank).

That's all you have to read as a beginner! the below section is for manual fire, for when you want to get your hands dirty and kill Ivans manually and more effectively.

Tip: Turn on save ammo order to reduce ammunition usage; as ammo can be expended rapidly by the AI, turning this on means the AI will only open fire if it suspects chance to hit is acceptable IIRC.

Manual Fire Designation Missions

This is where the chain of command MATTERS.

Lets start with the simple case: the Battery Commander (Nagel) is the spotter.
Lets say Nagel is doing not so great of a job, or you want a particular area where you believe Ivan is hiding to be targeted. Well, you can manually target that given area (provided you have LOS ofc). This is done by selecting the spotter, Nagel, then by selecting this button and right click (or right click drag) over an area you want to bombard.
Bombardment will commence after the accuracy percentage reaches +25% and will only stop after manually turning it off or until all batteries are out of ammo.

There are nuances to manual fire, whilst AI auto designation allows for all commanders (with established wire connections) and within the correct order of battle (i.e. cannot be a commander from another company) to ask for indirect fire, manual target designation for some reason requires the correct chain of command.

Explained with examples:
In this example, the spotter (in red) is the Battery Commander (Nagel), and the commander within voice range of the battery (in green) is a fire Platoon Commander (i.e. Elter), well in this situation Nagel can issue manual fire designation orders to the fire Platoon Commander who can then issue the battery to commence firing.

However swapping the places around, what if the Battery Commander, Nagel, is back in voice range of the battery (green circle) whilst Elter is the spotter (in red). If you try manually issuing an fire mission out of LOS for Nagel (i.e. over the hill), the percentage indicator on the area will stay at zero. Why? because Elter is a mere platoon fire Platoon Commander and cannot possibly issue fire orders manually for the whole battery.

To simplify this: Usually, only the Battery Commander can issue manual fire orders.

An example with fire platoons set apart:
In this example all Platoon Commanders have established wire comms and have clear LOS on the area of designated fire (the orange "100%" circle), the Battery Commander (cmd) does not. Given that the fire Platoon Commanders are within voice range of their respective guns, the target area will be fired upon by no.1 and no.2 fire platoons. This is because the fire mission is given directly to the guns, bypassing the chain of command.

KEY: When you select a particular fire Platoon Commander and designate a fire mission, you designate the fire mission for the whole battery, unfortunately you cannot have individual manual fire missions for different fire platoons.

So in this example, fire platoon no.3's commander is placed away from his respective guns, as a result, platoon no.3 will not open fire on this manual area fire mission since there is no chain of command for those guns to receive fire mission orders (they will still open fire on targets in the area using AI fire mechanism provided they are spotted by personally by the gunners).
Just to reiterate, if fire Platoon Commanders are not within voice range of their respective guns, ie Gottwald swaps places with Elter in no.1 and no.2 fire platoons, then no guns will open fire on the manually selected area. So please ensure you place guns with the right commanders.

To summarise, in order to ensure manual fire is fulfilled, it is best to follow the chain of command or to have the Battery Commander as a spotter (although if he is killed you will have no manual fire capabilities).

Why is knowing manual fire important?

I mentioned you can fire on un-spotted enemies positions you suspect, with manual fire you can also cheese fire missions. Whenever I start a tactical battle, I look visually for enemy trenches which will be full of Ivans when battle start, and since the enemy in those trenches are not actually "spotted" until contact is made, I can manually designate a fire mission directly on the trenches to kill whoever was located there. This is more effective than scheduled arty, which you have to rely on guess work to engage enemy positions.

It means you can make Ivan's trenches look like this:


A tip on scheduled arty: AI placement within tactical battles is predictable (they will always deploy within a 250m radius of their position on the operational map) meaning that you can cheese start the battle to see where roughly they are placed, then restart placement and then concentrate all your scheduled arty on that area roughly to avoid shelling those dangerous grass blades.

Final tip on manual fire: If you want to designate an area for smoke bombardment, you can select the Stay concealed order (crossed out eye icon), in which case the gun will fire its smoke shells instead of HE.

Good luck!