Question about .air file

I'm using AirEd 1.52, and I know that "0" in the "Aircraft Type" column means it's flyable, and "2" means it's AI only. But what does "1" mean? I noticed that my HoverControl helicopters have the "1" value.
Thanks!
(Oops, just realized this should've gone in "Adding AI & Gates". Sorry! It's my first time back in a long spell.)
Thanks!
(Oops, just realized this should've gone in "Adding AI & Gates". Sorry! It's my first time back in a long spell.)