This is out of my field, but I believe there are two different things both being referred to as skeletons here -

The first, the skeleton referred to in descr_model_battle, is just really a collection of animations (or rather a file that references different animations).

The second is something to do with being able to add completely different models (e.g. ones with eight arms and the like) which can't be added. A vet modeller will probably able to speak to this better.