Why can default supported nav agents only walk on Vehicle and Pawn?

I have no supported agents in my project settings and somewhere I read that in this case all agents are supported.
So I found out that when my floor mesh only “draws everything green” when I set the vehicle or pawn channel to block, else nothing will show up.
Now I wonder why that is. Is this NavArea_Default? And why is it determined by those Collision Channels?
I found no way to change this anywhere and don’t quite know what sort of object is related to it.