The "change unit flying height" trigger

Simple question - the trigger looks like this:

Animation: Change (unit) flying height to X at Y

Now, X = flying height.

But what's Y? Seconds?

How many spaces it moves per second until it gets to that height. I think it maxes out though, I've tried moving it only 40 but at 900/sec, and it wasn't instant.

So Y is how many "spaces" the Flying Height transformating should happen over?

What's/How big is - one space?

Y= how long in seconds it is going to take to get the new flying height set by the action.