JAR:
1. When you are relieving the Commander, you can write it in PIC. If you are in a pilot seat & the commander is also in a pilot seat you write it as Co-pilot time.
2. Augmented crew:
- if you are commander of the flight: log all block time (even if not at the controls)
- if you are any other position: only log time at the controls
Note: there can only be 1 commander on a flight, but there can be 2 captains.