WF also supports another kind of service known as a local service (sometimes called a data exchange service). This is a service that you design and implement yourself. A local service can serve just about any purpose, but one general use is to facilitate communications between workflow instances and the host application. In contrast with this, core workflow services each have a purpose (e.g., persistence, tracking) that has been defined by Microsoft. You can develop alternate implementations for each core service, but that doesn’t change their defined purpose.
A local service can be created via code or from entries in an application configuration file. There are three different ways to use a local service. 1. used directly from code in workflow class 2. used in custom activity 3. CallExternalMethodActivity