Instancing
Instancing is a mechanism in USD where layers can be designed to save memory and composition complexity. When a layer is for example referenced onto a prim, USD will re-compose the entire hierarchy for each reference in that layer, even if all references point to the same layer.
With instancing however, USD will create a “Prototype” of this reference’s composed result behind the scenes, and every reference to this layer is replaced by a local-reference to this prototype prim instead. Enabling USD to compose the referenced layer’s layer stack only once.
This concept does however come with a caveat, overrides defined on prims underneath the prim that holds the reference are ignored.
All selected prims in the example above reference chemistry_bottle01.usda, but are also marked as instanceable via the instanceable = true metadatum.
This metadatum tells USD that it should only compose the chemistry_bottle01.usda once in a __Prototype prim. When having done so, all references internally to chemistry_bottle01.usda are actually local prim references to the prototype prim (in this case __Prototype_37).
When a prim is marked as an instance, Stage Traversal will also halt at this prim!