There's more info in the online help ( Text Formatting Using
counters in autonumber formats) and manual, but briefly:
1. The M: identifies a numbering track, one of 54 the software
supports. Each is identified by an upper or lower case letter, or not
identified. Each track keep count of
The M is a series label. FrameMaker lets you define multiple,
independent numbering series which are distinguished by
single-character labels. Upper-case and lower-case letters are
different labels and you can also potentially use single digits
and punctuation marks if you need more than 53
