Stages are similar to Catalog Items in that they reference an asset from the Asset Library. Specifically, stages reference Scene Assets. Stages are meant to be the means by which the merchandiser collaborates with the artist to address product placement, lighting, position, rotation, camera angles, etc; essentially, all elements involved in staging the product.
Stages will typically require a placeholder to dictate where a product should be placed. Beyond that bare minimal setup, the artist is free to expose scene properties for manipulation in accordance with the attributes created on the Stage, via the same process as creating a configurator.