Maximum vertical clearance was a measurement of the obstacles a vehicle could clear.
In 2370, to aid low gravity dweller Melora Pazlar in accessing various areas of Deep Space 9, Julian Bashir modified the maximum vertical clearance of the Elaysian's wheelchair "three centimeters to a slope of fifty-six degrees". Miles O'Brien remarked that it would still leave many areas inaccessible to her. (DS9: "Melora")