Underload Check?
You mean the "Underload check?"
Underload + RTOW = Max Allowable Weight
MAW is checked with:
MZFW + total Fuel on board
MTOW
MDLW + enroute fuel
The smallest of the three is your MAW for the flight.
max allowable weight-wet operational weight = max payload
max payload-total payload = underload
Got the underload? Then cross check
Underload + RTOW = MAW
The underload is to tell you how much last minute changes you can have.
PK-KAR