What is this port above the waterline/return?

[exit]

Hopefully a quick question. Googled to no avail.

It’s a threaded plug, removing it and bubbles appear in the return flow.

So what is this thing’s purpose?

 

[mknauss]

It allows air to be pulled in through the venturi jet for that return. Creates 'spa' bubbles for that area.

 

[exit]

Thank you!