User`s guide

Description
This command selects the calculation function to be used. The default
function is NULL.
The null function provided in this command is for compatibility with
the 34401A 6½ Digit Multimeter. For the 34410A, 34411A,L4411A
it is recommended that you use the individual (null per function)
null commands found in the [SENSe:] subsystem, rather than the
null function provided by the CALCulate:NULL:OFFSet command.
Avoid using both types of nulls, as unexpected results may occur
(they are additive).