A bridge domain is being extended out of a Cisco ACI fabric. Between which of these components are policies used to identify the connectivity between Endpoint Groups and the external connection?

External bridge domains and external routed domains are concepts related to L2Out and L3Out configurations but are not the top-level policy components themselves.

While contracts are used, "internal contracts" and "external contracts" aren't distinct ACI policy objects in the same way L2Out/L3Out are. External networks use external EPGs with contracts.

External EPGs are used within L2Out/L3Out configurations to represent external endpoints, but the overarching policy components for external connectivity are L2Out and L3Out.

L2Out (Layer 2 External Network) and L3Out (Layer 3 External Network) are the specific policy constructs in Cisco ACI used to define connectivity between EPGs inside the fabric and external networks. L2Out extends a bridge domain as a Layer 2 connection, while L3Out routes traffic to external Layer 3 networks, and both define the policies that govern this external connectivity.