The bounciness combine mode used by the Collider2D.
A Collider2D can have a PhysicsMaterial2D assigned in several ways: directly through the Collider2D.sharedMaterial, indirectly through the attached Rigidbody2D.sharedMaterial, or by using the global default. If no PhysicsMaterial2D is assigned, a default value is used.
Additional resources: PhysicsMaterial2D, PhysicsMaterial2D.bounciness