The offset procedure is used both for approach and area air traffic control, although for different reasons.
In the case you've witnessed, approving the Saab to fly on an offset track (from the original one flown by the JAL 747) is just a mean of keeping him out of the turbulence of the preceding heavy 747. His departure track will be parallel to the jumbo's by x Nm. The distance from the original track can be either specified by the controler or proposed by the pilot and then authorised or not by the controller.
En-route, the offset is mainly used when you have 2 flights, following the same airway for some time and too close to each other to provide them with the same cruising level. Later on, their routes are going to split so, not to penalise the traffic at the lowest level, we "push" one of them on an offset track (5,8 or 10 Nm to the original one) in order to get them both the same level.
Let's say the original route is: xxxxx-yyyyy-zzzzz. The traffic on the offset track will then fly on a stricly parrallel route 5Nm (for instance) off xxxxx-yyyyy-zzzzz.
Hope I have answered your questions.
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