I just today noticed a fleeing AI cap carrier moving to a exactly opposite phase-line to where he phased in, while not evading the planet by moving besides it but above.
As it was the first time for me seeing that I was puzzled so I tried myself, and after 3-4 trys I figured out how to do it.

Well, it looks like in the standard installation the z-Axis-Movment has by default no keybinding to it. And unknowingly as I was, no mentioning in the tuturials, and not reading the manual I jumped to the false conclusions.
But anyway is z-Axis-movement used in Multiplayer or not? I´ve never seen it.

Where to (ab)use it?
My first thought was cruiser-carriers sitting on the top of suns (with hold-position setting) if you´re in a game with multiple star systems, as the 'invaders' tend to pop up randomly around you´re homeplanet-systems sun and you will in the end need less ships to defend against or weaken the incoming fleets as the all the bombers/fighters don´t have to fly around the sun. Additionally your force is gathered in one place (maybe along some repair cruisers) and not so easy hit&run-prey for flyby enemy fleets picking one of the all around scattered ships.
On second thought if positioned beneath the suns/planets, your opponent (if he has zoomed in from above the grav well) would need to turn his viewing angle all around to look at your fleet giving you time for micro/macro while he´s looking for the enemy.


One additional question in the end (kind of related):
I couldn´t figure out clearly whether the weapon range only counts horizontal or not.
(Can ships fire at something if it comes into its targeting range 'cylinder' OR 'orb'?)
The difference wouldn´t do much as you can´t normally move your ships above or beneath the flat layer of the grav wells, but there are exceptions as the one I pointed out or when ships fly around as group some fly 'higher' as the others.