pixels from the ground and up, Z height would be ground height + unit's flying height.
So it's not a rate, it's a value that represents one unit (one pixel) in Z axis
a value of 100 means the unit is flying 100 units above ground.
Edit 2: Confirmed that like I said initially it's not a rate as in there aren't any kind of multiplier (besides time). it's a flat increase/decrease in height per second. I did however neglect to mention that it's, obviously, from the initial height as of when the trigger is fired to the new height.