Maximum Output is the highest SPL (Sound Pressure Level) a loudspeaker can produce over an extended period. Burst, or transient volume is not measured as that number can ne quite high but is misleading. Proper measurement of Maximum Output is on the form of an average.
Typically, measurements are taken in the dBA scale. dBA is the international standard for measuring environmental sound pressure level. A-weighting compensates for how the human ear responds to sound (i.e. it is less sensitive to lower frequencies).