I am missing documentation on FET's

I am trying to work with feature creation using FET but I cant find any documentation on them. The TOOLKIT User Guide says that <creo_loadpoint>/includes should have header files with feature specific element trees but includes folder only has spgusrgl.h in it and that doesn't seem very useful for what I am trying to do. 

If anyone knows how to get the header files mentioned in the User Guide or has a better resource for understanding the FET attributes that would be greatly appreciated.