pxr.Usd.Stage
A stage is the USD abstraction for a scenegraph derived from a root USD file, and all of the referenced/layered files it composes. A stage always presents the composed view of the scene description that backs it.
The outermost container for scene description, which owns and presents composed prims as a scenegraph, following the composition recipe recursively described in its associated "root layer".
As described in its class documentation, a UsdStage is the interface to a specific pxr.Sdf.Layer (known as its rootLayer), interpreting the data it contains through the composition rules provided by Pcp. Pcp informs the UsdStage about which pxr.Usd.Prims should be populated on the stage, and provides PcpPrimIndex objects, per-prim, which allow usd to perform efficient value resolution.