Power Coefficient of Fixed Attenuators is a very complicated topic, it is defined as the attenuation change when the attenuator is input with 10mW power and up to its max rated avg power. For example, for a 2W attenuator with a < 0.005 dB/dB/W Power Coefficient, when the input power is increased from 10mW to 2W, then a 30dB attenuator would change in its attenuation by 0.3 dB (30dBx2Wx0.005dB / dB /W).

Please check the detailed test report at Power Coefficient Test for RF Fixed Attenuator