[help] What are "tags" and where are they ?

[Bifidus]

I keep coming across tags for various things in the .xml files and often they are self-explanatory, like what part is a texture, an animation, sometimes its uses or exceptions like collision, stuff like that.
But, for example, I'm looking at how seats works on bridges and I just don't get what does what. There's a few tags about pilot_sit/npc (wich works fine) but some others are not clear at all.
Is there a file somewhere detailing what they do ? I looked around a little bit and didn't found anything :/