I hope this helps. In new builds they are putting a kinda inset between the 2x4s just for the dry vent. It looks like a metal with a hole for you to attach the vent to. The “box” allows for more room for the vent hose to go in allowing more room for the dryer to move back. You don’t have to have a new build to use it, my house was built in 1996 and I just had 1 put in and it made a world of difference and allowed the dryer to go back farther. The piece in question is actually called a “dryer vent box” I hope this helps.