Just to begin with: there is a legacy avatar system that half the people use. It has clothing system etc. but it's ugly, so other people hide their legacy avata

Just to begin with: there is a legacy avatar system that half the people use. It has clothing system etc. but it's ugly, so other people hide their legacy avatar with "alpha masks" and wear "mesh avatars", which are meshes rigged to the same armature as the "legacy" avi. Then there are different rigging methods, the avi has a set of basic legacy bones, a set of volume bones, and they recently added another set of bones. Their market contains clothing and skins/meshes for all possible combinations of those hacks. What's even more confusing is that SL gives by default a "mesh avatar" to new users, and the poor guys have to learn about all this clusterfuck after the fact.