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:
OnCollisionEnter2D
OnCollisionStay2D
OnCollisionExit2D
OnTriggerEnter2D
OnTriggerStay2D
OnTriggerExit2D
NOTES:
This does not control whether the Collider2D will come into contact or not but simply if the resultant callback will happen.
Even if all callback layers are selected, only contacts captured via Collider2D.contactCaptureLayers, will be reported.
The other Collider2D involved in any contact callback disabled here will still receive callbacks defined by its own callbackLayers property.
Normally both the Collider2D and the Rigidbody2D it is attached to receive a callback therefore this option controls both those component callbacks.
When enabling callback layers where callbacks already exist, those contacts will not be reported as new contacts i.e. there will not be an OnCollisionEnter2D or OnTriggerEnter2D callback produced.
See Also: LayerMask.