A quick search the meaning of the abbreviations for the previous versions of the Soyuz spacecraft easily tells me their meaning:
Soyuz T. 'T' is for транспортный, transport.
Souyz TM. The 'M' is for модифицированный, modified.
Soyuz TMA. The 'A' is for антропометрический, anthropometric.
Soyuz TMA-M. What is the 'M'?
Soyuz TMA-MS. And what is the 'S'?
I know there are more than one designation system, for instance, reading through my notes, the TMA-M had the internal designation 11F732A47. But the TMA-MS name (or just MS) is in common use, so I believe the M and the S must mean something. I am not able to figure it out, so what is it?