Hi James, well one thing that's a little complex with it is that Separate can take in curves, faces or breps as input objects. For faces it will break the faces off of the brep but any faces that were touching each other remain in joined clumps.

So for example if you have a box and you select 2 adjacent faces and run Edit > Separate on it, it will break off those 2 as a joined surface, not as individual surfaces yet, but since it's now an independent brep and it's selected you can run Separate on that kind of selection to then break it into individual surfaces.

Do you care about that face sub-object selection behavior of separate or only the curve and brep behavior? The face selection part could get pretty complicated to document in the case where the faces from one brep form multiple disconnected chunks.

- Michael