MiniDisc Equipment Designation Information
'MZ-Rx' type models: Recorders.
'MZ-Ex' type models: Player-only units.
'MZ-Gx' type models: Recorders with radio remote.
'MZ-Dx' type models: Downloaders and lack any recording capabilities *"D" is used as an extension of the model number, i.e. MZ NH3D
'MZ-Nx' type models: NetMD capable.
'MZ-NEx' type models: NetMD downloaders.
'MZ-NHx' type models: Hi-MD units.
'MZ-EHx' type models: Hi-MD player-only units.
'MZ-DHx' type model: Hi-MD unit with camera module.
'MZ-RHx' type model: MZ-RH1 being the sole 3rd generation Hi-MD recorder. (*is also Mac-Compatible)
'MZ-Mx' type model: Mac-Compatible Hi-MD recorders. (*MZ-M10/M100; the MZ-M200 is essentially the MZ-RH1 with a bundled microphone)
*Note: models with 'F' suffix denotes units with tuner or tuner remote for listening to [but not recording from] the radio. *An example: the MZ NHF-800 is a NetMD capable Hi-MD unit with tuner remote.
The use of 'R' seems to have changed for the first generation of HiMD units, though this apparently will be back in use with 2005's models. [My thought on this is that 2004 'R' models are standard MD only; as Sony phases out MD entirely, HiMD will take over the 'R' designation.]
As for the numbers - they're more or less arbitrary. For a given model year, they're useful for distinguishing the base models from the higher-end models.
Try looking at the equipment browser on http://www.minidisc.org; if you trawl through some of the models there, you'll see what some of the differences are as far as the numbers go.