SAMtags.pdf §1.7 describes MM:Z as

MM:Z:…
The first character is the unmodified “fundamental” base as reported by the sequencing instrument for the top strand. […two further occurrences in later paragraphs…]

However “top strand” is a relatively uncommon term, and it does not appear in SAMv1.pdf's glossary. IMHO it should be briefly explained where it appears and/or in the glossary.