The pose library is a set of instructions on changing the figures positioning. People found that it could also change the materials. They can also, load or remove morphs, or set the values of morphs. So you have Poses for position, poses for setting morphs, poses for installing/removing morphs and poses for materials and combinations of those.

It helps if you understand that all the files in poser are basically subsets of the scene file.  Hair is often a conforming figure and so a lot of hairs wind up in the character(figure) library. You can move them into the hair library if you want.

You may have found the face and hand libraries. Those are often found in the pose library as well.

The material library is the newest way of storing and applying materials and will gradually replace mat poses. You also have 'mor' poses which apply morphs

It would be nice if everything was better organised but its not so you learn to deal.