All very interesting and this has been interpreted differently in different companies/countries for as long as I can remember.
Right now, JAR-FCL rules apply but they have the "out" that a system can/must be approved by the local JAA Authority.
One interpretation of this that I heard from one JAA authority was that the FO could log it as P1CUS "provided the FO made all the decisions regarding the flight including deciding on the fuel load etc at the planning stage and the Captain did not have to intervene to override the FO's decisions at any time." The Captain was also required to sign the FO's logbook to this effect, for this particular authority to accept the time as P1.
I hear that ICAO is considering a move away from all these classifications to a simple total hours-based system which would be good news for all.