Since Rhino creates, and stores, objects (geometry, text, values etc) it needs to store all that in some kind of storage. I have never seen it, not smelled it or stepped on its borders, but it’s in there.
But if you want to ONLY deal with that very storage mechanism without even opening Rhino, you can use openNURBS (I think) and it will provide you with useful code entries for properly accessing all that data. No need to know the internal structure or so, but that part is opensource (IIRC) so even that is possible if you need to know more details.
I think it means the in-memory version of the Rhino “document”. Probably written without regard for the more frequently used terms for it. At least that’s my guess.
That guide is definitely not easy to understand, despite it’s shortness