If it's like other Huma regulators, the setscrew has a tiny hole drilled in it. If you want, you can screw it into the front of the regulator so it serves as a flow restrictor. Installing the setscrew restricts the airflow and results in a bit better regulator accuracy and consistency, but it causes the plenum to fill much slower, which can result in inconsistent velocity on a quick follow up shot if you don't wait for the plenum to fully refill.
I always leave the restrictor setscrew out, ES & SD are still very good.
