Interestingly enough, LASORS 2008 (UK CAA) "Guide to logbook annotation" (Section A/Appendix B) states the following:
OPERATING CAPACITY
Pilot on flight deck but not as P1, P2, SPO or FE:
(i) Acting as ‘required’ Flight Navigator (under the Air Navigation Order);
(ii) Pilot supervising Co-pilot activities;
(iii) No duties assigned (Supernumerary);
DESIGNATION IN LOG BOOK
(ii) P2
RECORDING OF ITEM
Enter as for case C.
CASE C --> Co-pilot, P2, Enter time in ‘Second pilot’ or in ‘Co-pilot (P2)’ column.