6.4 truck engine - intake question

I had to remove the intake off my 2018 6.4 in a work truck. 2500 to be exact- vin “J”

Right behind the throttle body is a twist cap- so I twisted it. What’s it for? With the throttle body off I can reach in and feel a hole going from a passage in it to the plenum. The part that came out rattles likes pcv. Thanks! Syleng1

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