User`s guide

Remarks
You must select (CALCulate:FUNCtion) and turn on (CALCulate:STATe)
math operations before writing to the dB Relative Register.
The dB relative value parameter is relative to the dBm reference set with
the CALCulate:DBM:REFerence command.
The instrument resets the dB relative value to the default after a Factory
Reset (*RST command) or an Instrument Preset (SYSTem:PRESet
command), or when the math function or measurement function is
changed.