The Stainless Steel FLX-VP Cap is an alternative to using the Stainless Steel FLX-VP Barb as your seal between sampling events.  Once you have installed the Stainless Steel FLX-VP, the Stainless Steel FLX-VP Cap can replace the barb to seal the hole.  This is an attractive option if you are using a Swagelock fitting for your sample collection needs.  The  Stainless Steel FLX-VP Barb is still, however, needed for installation of the Stainless Steel FLX-VP.