What can I and what can't I use from the starter.fps folder?

I would just like know what I can use in my game from the starter.fps kit? All I'm using right now are some textures and some two sounds. The sounds, I'm going to replace with my own. The rain, water surface, ground, and particle textures, I would like to use.

It would be nice to be able to use the textures from trees, rocks, and some buildings. Maybe even some of the trees and rocks themselves. I could most likely create my own rain and particle textures, but I tried doing my own ground and water textures once and it DID NOT GO WELL! So I would like to know what I'm allowed to use.
Thanks.