If it is a peak value, P = V^2/2R = 1^2/100 = 0.01W = 10mW
10mW = 10*mW As we have powers, convert "10" to dB via 10*log10 = 10*1 = "10dB"
10 mW = 10dB*mW. As we are lazy sometimes, we write this mostly as 10 dBm.
When it is an rms value, the power V^2/R = 0.02W = 20mW, converting "20" to dBs gives "13dB", so together with the mW:
20mW = 13dBm.
1Vacpk = 2Vpp
1Vacrms = 1*1.414 = 1.414Vp = 2.83Vpp
rt(2) is the correction between rms value and peak value for sinusoidal waveform.