Z= rolling and it only comes to bear if you are in cockpit mode and keymapped the "roll" key
the banking of a ship while maneuvering is not defined by this speed but the max.bank angle
Strafing and reversing is also defined by other values.
In short: as long as you don't fly in cockpit view and found a use for the roll(which the game punishs you for) the Z axis is useless, shamefully this is a very common errorr in perception ( yes i am looking at you dear devs ) - though Ursus tested it by modifying the files to find out 100% for sure how it works - and it works in the way i explained.