How work COMMON, UNCOMMON, RARE and FORCED?
DerMeister, are you referring to items such as ARMOR, SHOES, etc. in the entity file?
I'm assuming you are based on context. Those values (COMMON, UNCOMMON, RARE, and FORCED) are the rarity of the item for an given civilization of that entity. The rarity value assigns chances for those items to show up being worn by migrants (in the case of clothing), as parts of armor by invaders, and I think also in the trade goods of merchants.
I know that it also used to influence the chance that your civilization will know how to make the item (have it be available to craft). This was the case years ago, and I don't know if that behavior has changed or not. The following was true back then, and might still be now, I'm not 100% sure:
MAY OR MAY-NOT STILL BE TRUE, I'M A BIT OUTDATED:
FORCED means that the particular item will always be available to every civilization of that entity (in every game the civilization will know how to make that item), and it will be treated as common for the sake of being worn.
COMMON means that the item will have a very high chance of being known by the civilization, but not guaranteed. Every now and then you might have a civilization that doesn't know how to make that item. If the civ doesn't know how to make it, you won't be able to craft it in Fortress mode, and it will be treated as (Foreign) in the equipment screen. Common items are also the most likely to be worn by randomly equipped individuals.
UNCOMMON is, naturally, less common than common, but there is still a decent chance that the civ might know how to make the item. If known, they are worn less commonly.
RARE means there is a very small chance, but still a chance, that the civ might know how to make the item. Very rarely someone from that civ might show up wearing that item (if the civ knows how to make it).
An outlier behavior of clothing and armor was that each civilization is guaranteed to have a minimum of one item of clothing that can be worn on each body part (upper body, lower body, head, hands, feet) as long as at least one possibility is specified in the entity file. So if you only had one SHOES item defined for the entity, and it was marked with a RARE rarity, it would default to always being available to the entity every game, because it was the only foot clothing option.
Hope that helps.