ICAO Annex II, Leaddie? You probably meant ICAO Annex I, which reveals all.
Here is the definition of “Pilot-in-command under supervision” from ICAO Annex I:
Pilot-in-command under supervision. Co-pilot performing, under the supervision of the pilot-in-command, the duties and functions of a pilot-in-command, in accordance with a method of supervision acceptable to the Licensing Authority.