when a flying unit moves it get to either descend or ascend up to it's full movement.
A unit with 4/6: 15 got 6 MV while flying to a maximum of 15 in height. Spending 1 MV action he get to move up to 6 inches forward, this also means that he get to either raise himself or lower himself up to a maximum of 6 inches.
So 6 inches forward = 6 height bands up/down.
To reach the full height of 15 the unit must spend all it's 3 MV actions to spend them in any way until he moved at least 15 Inches forward in some fashion. (he can turn and whatever, just as long as the inches up/down match the inches forward).
Of cource you don't have to move up or down, but the maximum is always your units MV while flying.