See Aluminum Tolerances
Aluminum Square Tube 6061-T6 exhibits above average corrosion resistance, good machinability, high strength, light weight, good weldability, and the ability to be heat treated for even higher strength, all at an affordable price. 6061 is the most commonly used aluminum alloy in the world due to its positive characteristics. It is typically used in aerospace, automotive, cameras, couplings, marine hardware, electrical components, decorative hardware, structural applications, pins, brakes, pistons, valves, and bicycle frames.
Tube is designed to act as structural support, whereas Pipe is designed to carry liquid/gas/material from one place to another.