From Page 10 in the manual. Possession – *A Robot has Possession of a Game Object if it is carrying, holding, or encompassing it. See rule <SG4> for details on Possession limits. *
From this https://www.robotevents.com/VRC/2018-2019/QA/151 you also indicate that moving a cap with ball may constitute possession depending on the amount to which the ball's movement is being controlled by the movement of the cap.
The training videos https://youtu.be/IpreV3qZKvQ?t=65 also indicate possession would include where a game object became stuck on or under a robot such that it moved with the robot.
With many of the ball intakes this year (even when turned off) a ball may become partially stuck under a robot’s intake, such that it would still come out if the robot were to drive backward but would move with the robot in forward directions. Does this constitute possession or is this a head referee judgment call?
-Thank you