In Async process ( in other words a long running process ), after the invocation of a partner process or service, the BPEL process continues to carry on with its execution process while that partner service completes performing its operation. This partner service may take couple of seconds, minutes, hours or even days to complete. The Async BPEL process then receives the result of the partner service via a callback service, when the partner service is completed.
HumanTask (Web service) is one type of such a long running
partner service which returns the task result when user completes the
task. In this case, BPEL use B4P extension, which stops the BPEL execution flow which contains the B4P extension activity and waits till HumanTask output to come.