There are special routes used when crossing the Atlantic. These routes change daily depending mainly on weather (winds aloft). The auto-routing algorithm may be instructed to act differently in relation to North Atlantic Tracks, depending on the value of the "NATs" option:
Please note that North Atlantic Tracks also have direction and altitude restrictions, so you should choose a flight level range which is available on the current NAT message (otherwise the route generator will not be able to use these routes).
[CLOSE]