The Layers that this Collider2D will report collision or trigger callbacks for during a contact with another Collider2D.
When a contact occurs between two Collider2D, each Collider2D will get a collision or trigger callback. This options allows you to select which layer(s) will produce a callback.
The ability to limit which layers will result in a callback can reduce the complexity of the script inside the callback so that it can safely assume only specific layers will be reported. There is also a performance benefit in not performing callbacks that are not required.
These are all the physics callbacks which are affected by callback layers:
NOTES:
Additional resources: LayerMask.