A cap is a complete Swagelok tube fitting assembly that is used to cap a piece of tubing. It includes a nut, a front ferrule, a back ferrule, and a fitting body that is blank. The blank body allows you to pressure test your assembly before installing it into your system. A plug is used to plug a port on a Swagelok tube fitting, and it has machined ferrules already inside. Like the cap, it allows you to pressure test your system.