A received level is the measurement of an absolute power, that corresponds to miliwatts for instance. So that’s dBm.
The fading margin is an attenuation, that is a relative difference in power. And that’s not an absolute power, that can’t be express in milliwatt. It is relative… So it’s dB. A certain power – 3dB, means the power (whatever value it actually was) is divided by 2.
-6 dB = power divided by 4
– 9 dB = divided by 6