The data sheet doesn't say but I would assume it refers to the amount of load placed on the output. This to some degree changes the internal feedback and hence gain/phase response of the amplifier. Full power I would guess is the measurement under maximim load, 1/2 power at half the maximum load and so on.
You can't program the power and the amplifier can't change it's mode of operation so it must be a reference to how hard you are loading it.