A coupon consists of two pieces of pipe, approx. 6" each long (any diameter or thickness) that a welder has to weld together to prove his ability to produce a sound weld. Coupon pipe size and diameter is determined by the welding procedure to be used or code requirement.
For instance, under ASME codes, a welder who welds a 2" sch.160 carbon steel pipe coupon (and passes) is qualified to weld on C.S. pipes 1" O.D. and up plus 2X the thickness of the pipe coupon welded.
The thickness of a 2" sch. 160 coupon is .344", so therefore the welder is qualified to weld on pipes .688" thick.