[Gmecm] What really is a 'mask'?

[Robin Handley]

I've never really known what a GM software 'mask' is - never having seen a
clear definition of it. I had been beginning to think, of late, that a
'mask' (e.g. $58, $8D) is a particular version of GM code, which is used in
different applications (and given different 4-latter designations e.g.
ANHT/AUJP) by changing the calibration data only. But now I'm not so sure.
Could somebody clarify?

Robin