Smaller ac duct inside vent?


I am in a newly built home and went to change the ac filters in the ceiling for the first time today. When I removed the filter there was a smaller duct next to the main one, with a valve-type closure (which was actually open) and a place where it looks like a smaller filter would fit (about 8" x 9"). Anyone know what this is, and what if anything I should do about it? The only thing I could come up with is that would be where you place one of those scented air fresheners they sell that you can normally attach to the larger filters....
Thanks in advance,
-G
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