A million service units (MSU) is a measurement of the amount of processing work a computer can perform in one hour.
It reflects how IBM rates the machine in terms of charging capacity.
The technical measure of processing power on IBM mainframes, however, are Service Units per second (or SU/sec).
However, that relationship disappeared many years ago as hardware and software evolved.
IBM publishes MSU ratings for every mainframe server model, including the zSeries and System z9 ranges.