"Bridge domain interface is a logical interface that allows bidirectional flow of traffic between a Layer 2 bridged network and a Layer 3 routed network traffic. Bridge domain interfaces are identified by the same index as the bridge domain. Each bridge domain represents a Layer 2 broadcast domain."
The physical interface can even join more than 1 bridge domain (up to 4096 per router). For example, connecting to VLAN 200 (also Bridge Domain 200) as well:
service instance 200 ethernet