Azure Service Bus Relay is a cloud-based service that allows you to host an endpoint that relays any request to a listener associated with that endpoint. The Service Bus Relay service enables us to build hybrid applications that are exposed in the cloud but hosted locally in your on-premises environment using Windows Communication Foundation (WCF). WCF relay bindings that permit BizTalk as a listening service on the relay point can be combined with WCF adapters to use BizTalk:

The WCF-WebHttp, WCF-BasicHttp, and NetTCpRealy adapters in BizTalk allow the sending and receiving of messages through Azure Service Bus Relay using BizTalk as the service broker between the on-premises application and the cloud-based application.