Long overdue, coming in 11.02 beta 2:
That's a new dataref that (finally) lets plugin authors tilt the pilot's head.
Previously this had to be done by manipulating the multi-monitor view offsets. That method worked (barely) but doesn't work with multi-monitor in v11. The new dataref is multi-monitor friendly.
(The new dataref is also FMOD-sound-friendly -- that is, when you roll your head over you'll hear sounds above you out your left ear, as expected. Monitor offsets do not do that - only the pilot's true virtual head position affects sound.)