Apparently the YM in Hoya archboldiana YM Excellent stands for York Meredith from Australia who gave the plant to Ted Green. Here is what Christine Burton had to say about it back in 2010:

“Due to Ted Green’s attaching the frivolous title of ‘YM-Excellent’ to the giant red one, in a manner that is supposed to indicate a cultivar, people appear to think it is a cultivar.  Friends, that big red Hoya archboldiana IS the only one of those “Archies” that exactly matches C. Norman’s holotype specimen. It IS Hoya archboldianavar. archboldiana. The YM-Excellent does not belong there. I doubt that “Jumbo Red” belongs there, but I could be wrong about that. What the YM-Excellent means is that Ted Green bought it from an Australian collector, whose name was (maybe still is) York Meredith”.