The additional Layers that all Collider2D attached to this Rigidbody2D should exclude when deciding if a contact with another Collider2D should happen or not.
The Layer Collision Matrix defines which Layers can and cannot contact other Layers. This property allows you to exclude Layers that all Collider2D attached to this Rigidbody2D should not contact.
When excluding Layers, all Layers that should be included are first included before finally excluding Layers. In other words, both including and excluding a Layer results in the Layer always being excluded.
NOTE: Because Layers can be included or excluded differently depending on the settings of each Collider2D instance, there is the potential for a conflicting decision for whether contact should happen or not when two Collider2D instances come into contact with each other. You can find the detailed rules for how Unity arbitrates this decision in the Collider2D.layerOverridePriority documentation.
Additional resources: Rigidbody2D.includeLayers, Collider2D.includeLayers & Collider2D.excludeLayers.