A cruise relief pilot, or second officer is on board for flights longer than the maximum for two pilots. At our airline they are not allowed to take off, land or be in either of the pilot seats below 20,000'. (I don't work for CX) While some airlines use two captains on long flights, all of our FOs are type-rated, and assume PIC duties when the captain is sleeping. Our second officers are typically right out of flight school. I have found that it makes for a good method for them to learn the operational aspects of airline work with out throwing them right into the deep end. A major advantage for the CA and FO is that they end up getting more landings.