Defining GK Possession

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Since it is unclear by the NFHS, you should apply the USSF ATR requirement:

12.16 GOALKEEPER POSSESSION OF THE BALL
The goalkeeper is considered to be in possession of the ball when the ball is held with both hands, held by trapping the ball between one hand and any surface (e.g., the ground, a goalpost, the goalkeeper’s body), or holding the ball in the outstretched open palm. Once established, possession is maintained
when the ball is held as described above, while bouncing the ball on the ground or throwing it into the air. Possession is given up if, after throwing the ball into the air, it is allowed to hit the ground. For purposes of determining goalkeeper possession, the “handling” includes contact with any part of the
goalkeeper’s arm from the fingertips to the shoulder. While the ball is in the possession of the goalkeeper, it may not be challenged for or played by an
opponent in any manner. An opponent who attempts to challenge for a ball in the possession of the goalkeeper may be considered to have committed a direct free kick foul. However, a ball controlled by the goalkeeper using means other than the hands is open to legal challenge by an opponent. The referee
should consider the age and skill level of the players in evaluating goalkeeper possession and err on the side of safety.