I've always read it as e.g. "MacBook Pro 9, revision 1".
That explanation doesn't make a whole lot of sense to me given that often times different sizes of the same generation will carry the same major number but different minor numbers-i.e. the 9,1 is the mid-2012 non-retina 15", and the 9,2 is the 13" version of the same.
Even further, the 2011 models were 8,1 for 13", 8,2 for 15", and 8,3" for 17." The "8" MBPs had several major changes-all got Sandy Bridge "Core i" processors and Thunderbolt, but there was an early 2011 and late 2011 "revision" that carried the same model designations. The late 2011s were a spec bump.
Mac Pros get even different, since for example the 2,1 is essential one specific configuration of 1,1(dual quad 3.0ghz, where all 1,1s were dual dual). Of course, the 2,1 had different firmware that supported those CPUs, although I doubt many "1,1s" still in regular service still even have their original CPUs
🙂 . AFAIK, the main difference between the MP 4,1 and 5,1 is in the firmware and the design of the heatsinks on the dual processor version. The 5,1 went through two revisions-2010 and 2012, with the latter using a newer processor generation, and Apple didn't update the model designation.
Reaching way back, look at the difference between the PowerMac 1,1 and PowerMac 1,2. They fundamentally used the same logic board with the main difference being that the 1,2 didn't have an ADB port soldered onto it(the pads are there), but the case uses a totally different color scheme and more importantly Apple put a G4 processor in it. Also of note is that with the release of OS 9.2, they pushed a firmware upgrade for the 1,1 that locked out using a G4 processor in it...