On long range flights with 4 flight crew i.e SIN-LHR 60AB are crewrest seats.
On long range flights with 3 flight crew i.e. BKK-LHR 60B is a crewrest seat.
It really depends on the length of the flight and the number of flight crew.
It has always been this way since we have Club on the upper deck.
Before this with club only downstairs they used 15AB.