First/Second Officer mean exactly whatever the employing airline want it to mean. In my company, First & Second Officers are essentially the same (same training, qualification, flying etc), the thing that separates them is experience (1500 hrs) and more importantly, salary (less for S/O – no surprises there!)
In the Cathay Cargo case, a S/O would probably be acting as a Cruise Pilot on a B747-400. The normal two crew is augmented by a third pilot to achieve a greater allowed Flying Duty Period. The job would probably involve sitting in either flight deck seat for long peroids in the cruise while other crew take rest, but not being allowed to do take off or landings until fully qualified as a First Officer (but that depends entirely on an airline’s policy, its not a legal thing)