The PhysicsMaterial2D that is applied to this collider.
If no PhysicsMaterial2D is specified then the Rigidbody2D.sharedMaterial on the Rigidbody2D that the collider is attached to is used. If the collider is not attached to a Rigidbody2D or no Rigidbody2D.sharedMaterial is specified then the global PhysicsMaterial2D is used. If no global PhysicsMaterial2D is specified then the defaults are: PhysicsMaterial2D.friction = 0.4 and PhysicsMaterial2D.bounciness = 0.
いいかえると、Collider2D で指定された PhysicsMaterial2D の優先順位が一番高く、次いで Rigidbody2D で指定された PhysicsMaterial2D、グローバルの PhysicsMaterial2D の順位です。
Additional resources: Rigidbody2D.sharedMaterial & PhysicsMaterial2D.