You can ask whatever questions you'd like, haha. That's what we're here for.
I think Lime described it pretty good one time, let's see if I can remember:
"Item layering is how you'd think it is", or something along those lines. Basically anywhere there's a joint on a person there's a new layer.
Most items require multiple layers (shirts and jackets, for example, have a "torso" and a "back torso", as well as two sleeves broken into "front upper arm", "front lower arm", "rear upper arm", and "rear lower arm". Together all those layers make up one shirt item).
Some items (like faces or backgrounds) only take up one layer.
Hope that answers your question!