MMT stands for Market Model Typology.  FIX MMT is a trade data standarization initiative and aims at implementing an independent trade categorization solution.  In addition, FIX MMT delivers an efficient operation solution for complying with trade flagging requirements raised by ESMA as part of the MiFID/MiFIR post-trade transparency obligations in RTS 1 and RTS 2. MMT is fully protocol agnostic, it can be used within any protocol.
Below are the adopters of MMT trade flags.  Where applicable, the logos are linked to the individual firms MMT documentation.