As title.
4RPL is the scripting language built into the game. It can affect the game in many ways - it's used to make the ERN factory in one of the story missions, to build land in another, and so on. It can also have more drastic effects, like playing in "first person".
Maps which use it (or specifically don't, if an author would usually have some scripting) will often use the tags to indicate that.
Playing in first person? How the actual [riddle]would that work in a top down tower defense?? Color me intrigued
Check out map #341 in colonies