This device is usually plumbed somewhere around the filter housing for easy access. You would usually connect the air bleed point from the head of the pumps or the bleed valves to this point to save you having to open up the inspection cabinet. This is to help prime the pumps so that you can avoid air locks in your system.
Assemble through the spa shell with silicone you can use a gasket instead of the silicone but not both together as this often causes the gasket to splurge out the side and would cause a leak.
O'ring and bleed screw included.