Correlation is used to enable external documents to join running process instances. Each step that receives input (a subscription document) can use a correlation. Each of these steps has a Correlation page in the Properties view. A message start event, a signal start event, or a receive task designated Allow this receive task to start new process instance can start a new process instance.

Catching message intermediate events, Catching signal intermediate events, and receive tasks NOT designated Allow this receive task to start new process instance also use correlations.

In a typical run-time environment, instances of many different process models can be running simultaneously. When a published document arrives in the run-time environment, the Process Engine must determine which of the running processes the document is intended for. You must provide a means of correlating a particular document instance with a particular process instance. This can be done in two ways:

When you create a receive task in a process model, you can specify that it uses either a correlation field or a correlation service for document correlation, or no correlation. Correlation IDs are stored and tracked by the Process Engine; for more information, see Tracking Correlation IDs.

Correlation services are written in the flow service editor in Designer, and establish or match the correlation ID used by a process instance. For more information about using correlation services, see About Correlation Services.