According to [1] some manufacturers apply this limit when both speed and altitude limits are reached, while other manufacturers disable tracking when only a single limit is reached.

The limits are 1,200 mph speed and 18,000 m altitude. Google tells me a 747's limits are around 615 mph and 14,000 m; Concorde's limits were closer to 1,350 mph and 18,300 m. Still, I'm sure if you're making an airliner you can get access to GPS receivers that will work even at maximum speed and altitude!

[1] http://en.wikipedia.org/w/index.php?title=CoCom&oldid=608893...

IIRC, the legal requirement is to shut down with both conditions are present, some GPS manufacturers simply found it easier to implement each restriction separately.

It is one or the other.