What was the significance of the plane selections made by the terrorists on September 11?
The primary significance of the plane selections made by the terrorists on September 11 was that they were planning to make long journeys--meaning that they had the maximum amount of fuel possible, which led to a larger explosion.
If you are on PLATO the answer would be:
All four planes were traveling long distances, so they would have significant fuel on board to burn in the crashes.