North Atlantic Organised Tracks.
Published twice daily, Eastbound and Westbound.
They are only valid during their published times.
"Half degree" tracks are organised tracks that permit aircraft with the appropriate equipment to operate more advantageous routes within the published track structure.
Any aircraft operating over the North Atlantic not on a published track is by default on a random route.
It is possible to follow the points of one of the organised tracks outside its published validity times, in this scenario this would be deemed a random route.
It is also possible to follow the points of one of the organised tracks but fly above/below the published levels during the published validity times, in this scenario this too would be deemed a random route.