Help with my sump

[alucas87]

Hi all. Im getting back into marine fish keeping after about a 10 year break, and i was looking for a little help. i have never used a sump before, and the one that came with my tank is a little different to all the ones i have seen online.

There is a little chamber at the back that water can only access from one cut in the top of the glass just next to the inlet. there is no exit for this chamber, so the water just sits still there. could someone tell me if this is where i should be placing mp protein skimmer, or does it have another purpose.