Every different clan will have their own preferences, but most follow the same general concepts;
Fair - Usually the base must be accessible at all times, meaning you can't have a closing door or a hidden teleport with a hidden path, or something that would fall under those lines. Along with this, there must indeed be a base to raid. This would be up to the condition of who is involved, but usually it's up to both parties.
Weapons and Vehicles - There's a general rule to not have hidden armories, multiple of the same power weapon (one shot kill weapons), no power ups (overshield etc.), and no trait modifiers or modded weapons. There's also regulations against certain weapons being banned with specific vehicles, for example; some clans prohibit using both a spartan laser AND a scorpion tank at the same time. This follows a few common sense rules, being that just because it's in the base, you could have a verbal agreement that it will not be used. Some groups don't even allow vehicles on the map, while others require multiple respawning vehicles at spawn.
Fundamentals - Every group, be they mercs or an organized clan, will usually carry their own ROE list (rules of engagement) detailing what they do and do not allow. As a general rule of thumb, these are always up for debate for exemptions on a case-by-case scenario. A common rule is that a base must have multiple obvious paths and rules for killing at spawn. Most of the time, a detailed list of what a base must contain is somewhere in there, so ask for a copy if possible. Long stretches of hallways with only one way in are usually something not allowed, unless they are unguarded/reasonably passable. I mean, it's a base, it must be something to defend, this generally consists of something along the lines of; an armory, a great hall/entry hall, barracks, meeting hall/conference room/war room, etc... A few I have made have either been very minimal to conserve on large open scale encounters, and some have been very tight knit corridors with sharp turns and multiple ways in and out. I've also seen some people take base maps and add a few things to give it a more liner path.

The things that newer people tend to forget is that these are organized beyond reason;
War - Generally, high ranking members or Commanders, or even sometimes councils, will get together with other clans and write out a full declaration of war or practice war. It's a lot of common sense, mixed with organized communication. Usually, once war or practice terms are declared and ROE's are exchanged and all agreed on, scouts from one group will come and survey the base before initiating any actual engagements. Most often than not, both sides will require a full 8v8 at all times, with people on standby to replace afc's and trolls or disconnects. Some of the basics while scouting require intermediate or high understanding of most, if not all rules, meaning that it will either be 2 mid ranking members, or 1 high ranking member or even the commander themself. After all terms are met and confirmed, the lobby will fill to meet the 8v8 or a similar requirement, and the defenders will have anywhere between 5 to 15 minutes to prepare the base, depending on what was agreed upon before hand. The attackers will have 2 hours to infiltrate and hold the base, and it must be held for an agreed amount of time, generally set to 30 minutes. A held base usually consists of more than half defenders being out of the base, while 3/4's attackers hold the base. Terms for a held base on time vary from group to group. Officially, if it is not a practice raid, a lost base may not be used again and must be considered lost. This rule is always the same in official raids. However, I am not versed enough in the old rules to know if you may reclaim a lost base or not, or if the attackers may use your base elsewhere. I do know that it's usually frowned upon to download and share official raid bases.
Alliances - When two groups make peace, they can form an alliance, and assist each other in raids, usually only in attacking to replace missing or idle members. This one is near borderline common sense, while still being up for debate between the two agreeing groups declaring war/practice.
Gametype - This one is pretty simple, it's always FFA with no time limit or kill score conditions. Most groups have enforced uniform rules, and differentiate pretty well. No modifiers may be used that effect damage output, damage received, movement speed, no bottomless clip, minimum 5 second respawn, limit one grenade, and must include the Assault rife for spartans, and the plasma rifle for elites (depending on ROE for defenders), sprint, and unlimited ability use.

Everything included in this guide is meant for people who are either brand new to the clan side of Halo or are just wanting to know the bare minimum fundamentals of how raids work on the community side. The history of clans in Halo is extensive and filled with many, many amazing and legendary feats, and along side all the wonderful community created rules, there are also a fair bit of people who's life goal is to see you suffer and lose. A quick google on raid related things in Halo will provide you with just about everything you need to know, and I just wanted to provide a way directly linked to the Halo: MCC client. If you have any suggestions or things you want to see in this guide, please do let me know, as it would help the average person understand how much fun it can be to be a part of the piece of history.