Operating
Mass - The Operating Mass is the APS (aircraft prepared for service) mass of
the aircraft and the fuel for the flight, Traffic Load is not counted. It can
be thought of as the wet Operating Mass as opposed to the DOM defined
earlier."
If you learn diagram 4-3-1 in the electronic notes this goes a long way to
solving all of the probable mass and balance 'definition' questions you're
likely to get in the exam.