aquamaxx protein skimmer (2) air inlets

[SeeFu]

Does anyone know why the aquamaxx protein skimmer has 2 air inlet's available, but one of them is capped off?

 

[Snowxcross]

I have a reef octopus and mine has the same as you stated above. I think it's for something like a CO/2 scrubber, where you would uncap the second vent and use it for the scrubber. In my case, i left it capped and just use the one for my scrubber.