The interesting ones were ambient.ipl which gives the in-game x,y,z coords for Zancudo and hippie camp ufo's. We were able to translate then into map coords, see the researcher toolkit thread
Aside from these textual ipl files, there should also be binary ipl files as well. @hakatox wanted to lead the way to finding them. Once we locate the binary ipl files, they will have coords for models too, like the alien egg. It will be the breakthrough we need to finally find it in-game
Binaries ipl's are in the files, but they are in .xmap files.