Farside for reference take a look at 121.543 which deals with the issue at hand.
Sec. 121.543 Flight crewmembers at controls.
(a) Except as provided in paragraph (b) of this section, each required
flight crewmember on flight deck duty must remain at the assigned duty
station with seat belt fastened while the aircraft is taking off or landing,
and while it is en route.
(b) A required flight crewmember may leave the assigned duty station--
(1) If the crewmember's absence is necessary for the performance of duties
in connection with the operation of the aircraft;
(2) If the crewmember's absence is in connection with physiological needs;
or
(3) If the crewmember is taking a rest period, and relief is provided--
(i) In the case of the assigned pilot in command during the en route cruise
portion of the flight, by a pilot who holds an airline transport pilot
certificate and an appropriate type rating, is currently qualified as pilot
in command or second in command, and is qualified as pilot in command of that
aircraft during the en route cruise portion of the flight.
First look at how paragraph (3) below is worded. This would lead me to come to the conclusion that rest/relief are one of the same.
(3) If the crewmember is taking a rest period, and relief is provided--
Second, to my opinion in this regards, the CAAS (CAA) system is flawed. Take a look at paragraph (3)(i) below:
(i) In the case of the assigned pilot in command during the en route cruise
portion of the flight, by a pilot who holds an airline transport pilot
certificate and an appropriate type rating,
The flaw is that the word "Type Rating" does not interface between the FAA and CAAS (CAA).
FAA and Type Rating.
A Type Rating on a pilot certificate means that the pilot took a CHECKRIDE from the LEFT HAND SEAT and is licensed as pilot-in-command on that aircraft.
A Type Rating can be issued on an ATP or Commercial certificate (In case of non airline operations but an Instrument Rating is required).
There is no such thing as a Type Rating for a co-pilot (P2 equal of CAA). A carrier trains a pilot as a co-pilot and so will his records indicate. However there is no notation on his or her pilot certificate.
CAAS (CAA) and Type Ratings.
A carrier trains a pilot as co-pilot. His CPL will indicate, co-pilot (P2) priviledges. As soon as this pilot passes the ATPL exams and gains the necessary hours, his ATPL license will indicate, Pilot-in-command (P1) priviledges.
Do you see the difference? In the FAA system the pilot has to be trained and checked in the LHS to obtain PIC priviledges. The CAAS (CAA) system does not.
I am not saying that the FAA system is better. But the word "Type Rating" has a different meaning.
What do you think?