Layer
Layers deliver a convenient way to manage and organize your scene while also providing a comprehensive live workflow. Basically, Layers are nested USD files with histories of their own. Added on top of this however is a live workflow allowing users to set each nested layer to be edited and viewed live. For example, lighting, content, and the environment can each exist on separate Layers (nested USDs). As things are edited within the Layers, deltas are stored allowing you to manipulate anything in the scene non-destructively. Additionally, those Layers can be set to live on an individual layer basis which will show updates in Realtime of other users working on either the layer directly or within the same root USD.
The effect of Layers (deltas, models and otherwise) are considered in a bottom-up fashion. The layer above takes the priority over the layer below on any alterations. You can move Layers up and down freely by dragging them, to give priority.