I wouldn't call it an "addition" exactly.... It could be used in conjunction with it, as the overview points out, with the Abc files being used as the leaf node

I wouldn't call it an "addition" exactly.... It could be used in conjunction with it, as the overview points out, with the Abc files being used as the leaf nodes for the actual geometry representation. But the SceneGraph representation (USD) will need to be the starting point, as Alembic doesn't (yet) provide a nice way of doing overrides or referencing other external files which is what this spec provides.