The composed result of opening a layer. Think of it as a Photoshop canvas when opening a .PSD document. All the Photoshop layers interact with one another to produce a “final image”. The layers still exist, they can still be edited, deleted, replaced, etc… but what you see is the result, and that is the Stage in USD.
This composed result can be “flattened” and saved to a new layer, which would be similar to taking the aforementioned Photoshop document and exporting it to a JPEG.