I can't speak for anyone else, but low is under powerlines, and when the wheels bounce off the ground. Low is looking up at trees and houses and big rocks. Low is when bird strikes occur because they don't hear you coming until you're nearly on them, then they rise up out of the grass. Low is having to climb to clear a fence or a car.
When I was a kid, I was working a field with my boss. I ran out of chemical, and he asked me to climb to 500' and wait while he finished up. I remember being afraid of climbing that high.