You're right, there's not much I can find on the net either. So I'll fire from the hip here - can't guarantee this is accurate.
it sounds like you have an internal power filter for filtering/circulating water, which from your description it sounds like it does well. The stopper thing you're referring to my guess is that it's a venturi style intake to allow you to inject CO2
or soemthing like that into it if you needed to. most users don't need this function, so you should keep the rubber stopper capped to prevent air ingestion through the filter. That feature would only be used in specific cases.