Quite a clear anwser. My thanks for that.

grain, dyes, silk, crusade
Resources (and hidden resources like being crusader target or ability to have a guild) of the province
I was looking through the list of where resources are located, compared to where resources are mentioned in this regions_desc.txt file, I noticed one discrepancy: Constantinople is the only place with grain mentioned in the resources list.

Furthermore I can't really place the Jihad and Crusade tag, for I have seen Jihads held against certain targets as well as crusades.
It's also not a tag used by the AI, for I have also seen the AI hold jihads and crusades against these targets. So what does this tag do ?

As to the other tags and correct me please if I am wrong.

atlantic: In these spots you can build ships that can cross the atlantic.

(Toledo seems to be missing this tag though)

america: Well that's the other side of the ocean.

arguin: special region for Arguin. Can also only be reached by ship.

guilds are also mentioned, however it does not seem to be that in these cities those guilds appear, so what does the mention of the guild do there ?

I've got some modding experience, just not with any of the Total War series (well except for tweaking unit stats) and I'm trying to learn what stuff means. More questions will come, although I think that this is perhaps the wrong subforum to ask them.

Dingo

Aspiring Total War Modder