Question about terrain textures

[JKH]

I notice that some campaigns require desert textures. not being to familiar with them, if I d.l. a desert terrain will it replace the original terrain totally or will it only apply the terrain when I play a specific mission that requires it and then automatically go back to the terrain that came with the game when I play a standard mission?

[Placebo]

Totally new and seperate as you correctly state in the latter part of your post