Each layer internally keeps track of a “local” stack of layers that contribute to that layer. This stack is an ordered collection of layers used within this layer that contribute to its hierarchy and composition.
In the above, the layer
furniture_workbench01.usda is made out of three other layers and itself. It adds the layers,
furniture_workbench01_rigging.usda in that order to its layer stack. Visually this order will look reversed as it is a stack after all, but the higher on the stack, the more “important” a layer is.
A layer’s Layer Stack always has its own data at the top of the stack, making itself the most important.