HP 3326A Operating And Reference Manual page 97

Status Byte
MASK
The MASK command has the syntax of MASKnPC where n is an integer corresponding
to the enabled bits in the status byte, and PC is the suffix used to end the command.
The integer is determined by summing the decimal values of the enabled bits in the
status byte. Figure 3-1 describes the HP 3326A status byte and lists the decimal value
of each bit position. The require service bit (bit 6) cannot be disabled and mask program-
ming for this bit is ignored.