That tab restricts the amount of air getting pulled in I would assume.
What are you looking for specifically?
The orange tube on the other side of the skimmer controls the water flow. I don't know about this specific skimmer but open usually makes less water flow into the skimmer and closing allows more flow inside the skimmer raising the water level inside the body.
Opening the restriction on the air intake allows more air for more foam.
I'd open the air intake fully, then adjust the water flow to keep the foam sitting at the top of the throat inside the skimmer cup for a decently dry skim. Then, adjust the air to get the bubbles looking good and readjust the water flow as needed.
It might take a few days or even a couple of weeks before it gets settled and starts getting a consistent skim.
I hope that all makes sense?