You can also enter an optional conversation ID value in the Conversation ID field of an invoke activity (and other activities such as a receive activity and the onMessage branch of a pick or scope activity).

The conversation ID identifies a process instance during an asynchronous conversation. By default, the BPEL process service engine generates a unique ID for each conversation (which can span multiple invoke and receive activities), as specified by WSA addressing. If you want, you can specify your own value for the service engine to use. Conversation IDs are implemented with the bpelx:conversationId extension.