SCPI OUTput:COUPling command parameters AC|DC
Notice that section 7.1 of the SCPI-99 standard mentions "character program data" as a type of parameter for a command or query. Unfortunately it simply refers back to the IEEE 488.2 standard for the definition of this data type.

In practice, many instruments require character program data parameters for various operations. These often reflect an enumeration of different inputs, outputs, or modes of the instrument. For example, an oscilloscope I use regularly has this command for setting the trigger source:


  :TRIGger:SOURce {FPANel | FRUN | LMODule | RMODule}


The enumerated parameters typically are not mapped to integers as an alternative form of input. In fact SCPI specifies in section 7.2 that numerical program data "shall not be used for selecting functions on a “One of N” position switch."

The :OUTPUT:COUPLING command isn't even the only command of this type that is specified in the SCPI standard. Just looking briefly, in section 9.2.1.2.2 they specify a command


  :TRIGger[:SEQuence]:SOURce {BUS | IMMediate | ...}


and in 8.2.8.2 there is


  [:SOURce]:FREQuency:MODE {CW | FIXed | LIST | ... }