From everything I can gather, I think that is the range the plane can fly, 200 mile round trip, or just 400 miles there and land to refuel or crash.
That is a standard designation. It means you can fly 400 miles in one direction with no combat along the way. Combat EATS up the reserves FAST!
A better way to put is is in Hours of opperation @ __ Rpm @ __ altitude. All (or most anyway) planes have better gas milage at 15,000 feet than at sea level because the air is less dense. But the Horse Power and Max speed vary according to the plane and the altitude.
So, the easy answer is, range means take off and fly till you crash and that's 400 miles, more or less.
I never used range or asked myself questions about it since it is not relevant in CS, except for flavor.
Well that's another thing I been trying to think of a way to possibly deflate fuel.
That is say....a fighter has a range of 400 miles, has a 300 mile combat patrol circuit. Somehow rolling a die to determine what part of that 300 mile circle they engage the enemy, thus somehow maybe evaluating fuel levels.
There really should be a game mechanic for range, but I think that the weight of fuel for your range should be part of the construction process.
IE: a plane with a 400 mile range would need fuel weighing half as much as a plane with an 800 mile range.
I think that factoring different engines sizes would needlessly complicate things even though logicaly a small plane can go further on less fuel.