Everything you can wear can be attributed to a layer, a sort of virtual slot where you can fit a certain number of items, often just one.
Every time you try to wear an item but can't because you are wearing another item these two items are in the same slot. Sometimes you can wear two or more items of a kind before reaching the limit.
Items that are held like weapons and shields or just carried can also be said to be in a layer of their own. The primary purpose of those special layers is not to describe what individual items fit in but rather to specify how many items they can contain, what type of items can be held, etc.
Sometimes items that conflict do not cover the same zones and so you can either wear an item that covers many zones or instead several items that cover only some of those zones. For example, you can wear a breastplate and a backplate together but if you wear a cuirass you can't wear either. We can say that the breastplate and backplate are in sublayers of the cuirass.
The layer names are arbitrary, usually named for some items within it or their purpose.