この collider/rigidbody が他の collider/rigidbody と触れ合うのをやめたときに OnCollisionExit は呼び出されます。
In contrast to OnTriggerExit, OnCollisionExit is passed the Collision class and not a Collider. The Collision class contains information about contact points, impact velocity etc. If you don't use collisionInfo in the function, leave out the collisionInfo parameter as this avoids unneccessary calculations. Notes: Collision events are only sent if one of the colliders also has a non-kinematic rigidbody attached. Collision events will be sent to disabled MonoBehaviours, to allow enabling Behaviours in response to collisions.