XE Sky Variations.esp is helpful for deciphering sky/fog/ambient/sun colour functions but otherwise I'm at a loss how to interpret the usage (if indeed they are usable) of other functions defined in the MGE-XE customfunctions.dat file. Interesting sounding functions include texture (MGESetMeshTexture, etc), mesh (MGELoadMesh, etc) and I/O related (MGEPushKey, etc) ones.