1. When configured as per call, a new WCF service instance is created for each method call from the client. The instance serves the request and is then destroyed. 2. When configured as per session, a single WCF service instance is created and persists for the duration of a client session, serving multiple method calls. 3. When configured as single instance, a single global WCF service instance is created to serve all method calls from all clients. This instance persists for the lifetime of the service.