Let's use a real-life example I've done many times in the past. When you're departing from a smaller airfield that lacks the normal clearance, ground, tower, etc services you will contact the Flight Service Station (FSS) via radio or telephone to get a weather briefing and file your flight plan. They'll pass on the appropriate details for your flight - departure frequency, weather, changes to route of flight, etc...when that is done and everything is sitting good they'll give you a clearance void time...ie - be off before that time or your clearance is void. In PF3 they use the phrase "we'll open your flight plan at 10" - i just treat that as a clearance void time and it gives me about 10 minutes to get off the ground and contact departure/center/whomever.
In flight sim "reality" contacting FSS serves the same purpose but there is not really a clearance void time or anything like that - it's just words. If you take 30 minutes or 3 minutes - it's all the same. PF3 has your flight plan active.
Fly safe.
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Flying the friendly skies of MSFS
XBox handle: mikejenlyons
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A Pilot's Life 2: Michael Lyons
-- mike