C900 Low power

Does anyone know what the TxPower@DAC setting means? While troubleshooting low power problems with 900 ap and sm’s I checked the calibration information by http://xx.xx.xx.xx/mac.html and found that the ap and sm that have normal power have a TxPower@DAC28 = 28dBm and the units we are having problems with are TxPower@DAC21 = 21dBm. This seems to track with the differance in the power level we see.