lobby,
Since you are an air traffic person, I'll explain the flight plan;
There are a number of times when RIF is used. Basically the aircraft gets airbourne for one destination but knows in advance that for some reason the flight may proceed to another aerodrome.
The one I have come across before is on long range flights - one files to a destination that is conservative with regard to fuel and ensures reserves then if one has sufucuent fuel at the decision point, one can opt for the revised destination included in the RIF.
Example - old Atlantic flights eastbound when fuel was tight could file say Shannon as destination with Cork as alternate and then in item 18 specify;
RIF/54N15W Nxx STU G1 EGLL
This means that if there is sufficient fuel available at the oceanic boundary, the flight could (subject to ATC clearance in this case) be extended to Heathrow. If not then the flight refuels in Shannon.
Up until the RIF is acted upon, the flight is planned to Shannon and Shannon expect the flight. After the RIF is used, Heathrow becomes the destination and that is basically that.
These days the fuel available trans atlantic isn't a problem except for light aircraft ferry flights where the RIF can be used to overcome the legal reserve requirements i.e. flights on the atlantic must have large reserves.....thus one files as far as Shannon or Prestwick and then once across the atlantic, the reserves are not so critical (one needs 45 min as opposed to hours) and one can use the spare fuel to reach Paris or the UK or as far as one wants to try and push it with little sleep!
There is also the old fogbound airfield (below minima) - one files to a nearby airfield and RIF to the fogbound one if the weather clears.
It's all about passing on the intentions prior to flight thus keeping everyone in the picture.
Seems that it is also a neat way of ensuring that UK ATC can't simply bin a flight plan that one has addressed to them
Regards,
DFC