1.dBW + dBW = dBW

dBW - dBW = dB

dBW + dB = dBW

dbW - dB =  dBW

dBm + dBm = dBm

dBW + dBW = dBW

dBm - dBm = dB

dBM + dB = dBM

2.dB means Decibel  which is 1/10th of Bel.

3.30 dBm = 0 dBW

100 dBm = 100- 30 dBW =70 dBW

4.The source where the power is uniformly distributed to the surface that source is called isotropic source.