HP 83751A User Manual page 108

Instrument State Keys
If the special function consists of discrete states (i.e. On/Off or Int/Ext/None),
then the @j ($J keys or the front panel knob must be used to select the
desired state. If the special function is a numericaI value, then the keypad
may also be used to enter the value.
See Also Chapter 15, "Special Functions"
8 - 9