An external system (probably but not necessarily a BPEL process) that enacts a Cúram workflow via web services will often require notification that the process completed and possibly some output data from the process definition. Doing this requires a web service that will be invoked when the process completes to be specified for each process definition.
The callback web service is specified in the process definition metadata using the PDT or directly in the metadata as described in Process Definition Metadata.
The callback web service must be implemented by an external system but conform to a port type definition specified by the Cúram workflow web service, Invocation from BPEL processes has further details.