If the pigtail is left open to air, you would have this connected to continuous suction. If the pigtail is closed, should only be connected to intermittent suction to prevent damage to the gastric mucosa from continuous vacuum and no release. The pigtail (usually blue, at least it was when I was working) is not used for suction or to administer feedings or meds. The opening at the top of the main tube is used for feedings and suction or just left open vented. You may or may not have to use a Christmas tree connection to attach to suction. That will depend on the size of the connecting tubing.
Anytime you face a challenge you are not 100% sure of, ask another experienced nurse or your instructor. Never guess, it can be very bad news. Thanks.