ScenegraphXML is provided with Katana as a simple reference format that can be used to bring structured hierarchical scene data into Katana using a combination of XML and Alembic files. One example of how it can be used includes using Alembic to declare some base 'building block' components and then use XML files as 'casting sheets' of which assets to bring in.
ScenegraphXML files can also reference other ScenegraphXML files to create higher level structured assets. For instance you could have a multiple level hierarchy where buildings are built out of various standard components, and sets of buildings are assembled together in to city blocks, and city blocks are assembled together into whole cities.
In the hierarchy created by ScenegraphXML locations can be set to be assemblies or components to help users navigate the scene graph. By default any reference to another ScenegraphXML creates a location of type assembly and any reference to an Alembic file creates a location of type component. You can also override this behavior by directly stating the type of any location in the ScenegraphXML file.
You can also register proxies at locations, and create LOD groups.