when do u go for BPM ?

-->If the two Interfaces Sender & Receiver of complimentary modes(when more than msg).
-->When the msg will be casted to multiple receivers at the same time.
-->when the msgs is casted to the multiple receivers one after another sequentially.

What is an abstract interface?

A:Abstract interface is similar to other interface (outbound or inbound) but does not have direction. It can be used as either outbound or inbound. It is used only inside BPM.It is also called as hidden Interface