This is an old revision of the document!
I'm not sure how one first gets exposed to path variables, only that it can be confusing at first glance. This document tries to remedy that without going into the history of everything.
What you need to know:
There are various directories that follow this pattern [not limited to]:
bin: binaries (actual commands you execute: e.g.
include: source code
share: other, usually documentation
man: manual pages
The directories can exist inside of any directory path. There are the usual ones you may have seen already: