You can create an onEvent branch in a scope activity that causes a specified event to wait for a message to arrive. For example, assume you have a credit request process that is initiated by a customer's credit request message. The request may be completely processed without the need for further interaction, and the results submitted to the customer. In some cases, however, the customer may want to inquire about the status of the credit request, modify the request content, or cancel the request entirely while it is being processed. You cannot expect these interactions to occur only at specific points in the business processing. An event handler such as an onEvent branch enables the business process to accept requests (such as status request, modification request, or cancellation request) to arrive in parallel to the primary business logic flow.

The onEvent event handlers are associated with an enclosed scope. The onEvent event handlers are enabled when their scope is initialized and disabled when their scope ends. When enabled, any number of events can occur. They are processed in parallel to the scope's primary activity and in parallel to each other. Message events also represent service operations exposed by a process and modeled as onEvent elements. Event handlers cannot create new process instances. Therefore, message events are always received by a process instance that is already active.