Before starting each flight, the captain will decide which pilot will take direct responsibility for flying the aircraft this pilot will be the 'Pilot Flying' (PF) for that sector or the specified flight.
The other pilot will then be the 'Pilot Monitoring' (PM) or alternatively as 'Pilot Not Flying' (PNF) and in that role must monitor the flight management and aircraft control actions of the PF and carry out support duties such as communications and check-list reading.
The Operations Manual will specify fully the roles for the PF and PM/PNF, but one of the most important aspects of the duties of any PM/PNF is the cross-check of the actions of PF. Indeed, this part of the role represents one of the most important single reasons why a two-pilot flight crew is specified.
Subject absolutely to the determination of the captain, the two roles during any particular crew duty will normally be shared so that both undertake PF and the PM/PNF duties. Whatever their role, the designated aircraft commander is responsible for all aspects of the safe operation of the aircraft. Appropriately qualified Check/Training Captains may occupy either seat as aircraft commander for crew training purposes.
When the terms PF and PM /PNF were first introduced, it was normal for the roles to be designated, subject only to limitations on the ability to directionally control an aircraft on the ground imposed by the absence of a steering tiller for the pilot in the right hand seat, to be designated for a complete flight sector.
Nowadays, some operators use the Monitored Approach concept in which the pilot who is PM during the descent and approach will become PF for the landing in order to improve the operational safety aspects of the transition from instrument flight to the visual reference required for touch down after almost all approaches.