Used for 64-bit multimedia operands with MMX and SSE (XMM) instructions.
MMWORD is a type. Prior to MMWORD being added to MASM, equivalent functionality could have been achieved with:
mov mm0, qword ptr [ebx]
While both instructions work on 64-bit operands, QWORD is the type for 64-bit unsigned integers and MMWORD is the type for a 64-bit multimedia value.
MMWORD is intended to represent the same type as __m64.