Use a plug to close off an unused port in a valve, manifold, or HPLC column. The standard stainless plug consists of a zero-volume nut with a ferrule made from a piece of solid rod.
Use a cap to close off a tube that already has a ferrule and internal nut. A cap is essentially a piece of hex stock with a zero-volume fitting detail machined into it, minus the through-hole.
For more information on stainless steel, refer to Properties of Metals
