I think the tube you described could be considered Extruded Aluminum, and I believe there are no limits on this. As for the difference between chain and timing belt... well... chain is made up of different links connected together such that each link is solid, but the entire length is flexible. Belt is a single unit, not many connected units, the entire length is flexible. It comes in many different shapes and sizes. (ex. V-Belt, timing belt...)
I think those are pretty good definitions??
