Tim, and Nathan,
Under the new D1.1:2015 farm code, the tubing requirements for qualification and production are in Clause 9. Going mostly from memory on the old requirements from Clause 4 a 1G on tubing/pipe is rotated per D1.1. Otherwise it would be a 5G. So, your pipe is in the horizontal position with the circumferential groove and following weld going all the way around and encompassing every position, except 6G, and needing to be rotated during welding to keep the weld within a 15° rotation from true top dead center flat.
As such, position would be restricted within the limitations on the Welders Performance Qualification Record and that is the only position you could weld in. Just as using a 1G for plate and being limited in the weld being truly down flat and not horizontal, vertical, or overhead.
Brent