What is option H12 for the E4418B and E4419B EPM Series Power Meters?

Option H12 for the E4418B and E4419B EPM Series Power Meters provides an improved power level accuracy specification of +/-0.00882dB from -15 dBm to -10 dBm and +/-0.00668dB from -10 dBm to +5 dBm.