This output is available for HAM radio transceivers only.
It is a fairly complete implementation of the XML-RPC server that fldigi expects for rig control. While you can use the rigctld output with fldigi as well, the XML-RPC implementation is much more robust, faster and flexible.
In essence, this lets Wizkers emulate the 'flrig' program, so any software that is compatible with flrig will be able to work with Wizkers.
There are no specific settings for the XML-RPC output, just create it, then enable it in the "Outputs" screen, fldigi will automatically connect to Wizkers as soon as it starts.