You're thinking of HyperMAC (as opposed to SuperMAC, which is what Sony called the 100 Mbps version before it became an AES standard); strictly speaking, the 1 Gbps version has never been incorporated into the (royalty-free) AES50 standard, so "AES50" refers only to the 100 Mbps version.
-Russ
Thanks, yes, you're right, it's SuperMac and HyperMac (I prefer the weird capitalization to be optional).
As pointed out by Helge, Midas uses the HyperMac in their larger consoles; I thought it was in all the consoles that can handle 96kHz sampling rate?
There was a paper by Midas describing the two formats and their differences, which I don't have on this computer and don't have time this week to look up.
One thing I was wondering and sort of alluded to: The X/M consoles' AES 50 can carry 48 audio channels each way. Is there any combination of use circumstances that uses that channel count to full capacity both ways? I can't think of any.