Command time is Command time, and is logged by the Commander.
Being qualified as a Captain in an airline qualifies you to log Command time - if you are in Command.
If you are not in Command, it is possible to be In Command, Under Supervision (ICUS) and, if that is the case, you can log the time as such. This has nothing to do with who is PF for any sector. You do not decide after the fact if you were acting In Command, Under Supervision (that's not possible). If you are to act in that position, then it needs to be agreed before the flight with the Commander, briefed to any other crew (i.e. Cabin Crew), and all of the Command decisions will then be made by the ICUS pilot - fuel, delays for maintenance, tech log & MEL decisions, weather delays or in-flight diversions, Cabin Crew briefings, etc etc. The entire day would be logged as ICUS - regardless of who was PF on any particular sector.
As described, all of the time would be logged as FO time (P2, Co-Pilot etc)