TOPIC: plumbing vent question, please help

Has anybody seen this type of cap on a vent that does not extend the exterior?
It is located in a garage for a washing machine drain

Air Admittance valve, mechanical valve that will open under negative pressure from the line and will allow air into the vent, valve closes by gravity. They should be accessible as they are mechanical devices and may require repair/replacement from time to time. If in the attic they should be 6-8" above insulation. Allowed for this type application in most areas, Illinois may not allow if I am recalling correctly.

Allowed in some areas, not allowed in others.  Only your AHJ can answer the question.