61.085 Definition of flight time as co‑pilot for Part 61
A person’s flight time as a co‑pilot is any period, during flight in an aircraft that, under these Regulations, must be flown with a flight crew of at least 2 pilots, in which the person is performing co‑pilot duties other than as pilot in command under supervision.
Note: A co‑pilot is a pilot on board an aircraft in a piloting capacity other than the pilot in command or a pilot who is on board the aircraft for the sole purpose of receiving flight training: see the definition of co‑pilot in Part 1 of the Dictionary.
61.090 Definition of flight time as pilot in command for Part 61
A person’s flight time as pilot in command of an aircraft is the duration of a flight for which the person is the pilot in command of the aircraft.
I agree it's nebulous, but the bit about "
performing co-pilot duties" might get you into trouble if you log bunk time as flight time. It's different for the pilot in command, who can log the whole lot whenever he or she is the designated commander of the flight.