Earlier in the comments I hypothesised that force 999 is used by neutral npcs.

After some testing, I am reasonably sure that force 2 is used by NPC villages.
Force 2 and/or 3 in some combination are used by the npc tenants (survivors from the beacon).

With the current mod version where all 3 races have which doenst actually do anything (player is friendly to player..), you will find you are able to hurt your own town's npcs, and are also able to hurt friendly npcs in towns where you normally trade.

I think the normal setting is:

However, if being able to kill the friendly NPCS is a feature of the mod, then leaving this field empty would be okay.

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I have had a thought that having a custom race be on a team that is not 1, can create PVP functionality o.o but that is a idea for another mod probably XD