Good question. Assuming we're talking about Dan ranks, from what I've seen (generally speaking), fourth or fifth Dan is usually considered 'Master'. So those could be assumed to be instructor ranks where the person is moving (ideally) from an assistant instructor up to a master instructor. And I'd offer that by these ranks, everything has been learned in the system. Now some may require a kata for Dan ranks above master, but I'd offer that pretty much everything has been learned and 'mastered'. So any rank above the 'master' Dan rank (whatever that is depending on the art) should be more of an administrative rank that should depend more upon total time in the art, what you've offered back to the art, perhaps the quality or quantity (or both) of students taught etc. Otherwise, in most arts, what is the difference between a 6th Dan master and a 7th Dan master besides TIG/TIA? Shouldn't there be something of more substance?
Just some thoughts.
As far as Grandmaster. Well, shouldn't there only be one in the art? Why would we need more than one. At what point does it change from the need to establish a hierarchy to one of plain ego.
I became a GM when my instructor (who was the GM) retired and hung it on me. Too me, it is an administrative title to establish the hierarchy of the association and nothing more. I don't particularly use it much except for the official stuff and that is more for them than me. Well too be honest, it's all for them. I prefer Sensei.
But then Dr. Professor Ultimate Supreme Grand Master Ph.D. MA has a nice ring :rofl: