Found this in an article about BPEL (albeit from 2002):
"Web services implemented as BPEL4WS processes have an instanced life cycle model. That is, a client of these services always interacts with a specific instance of the service (process). So how does the client create an instance of the service?
Unlike traditional distributed object systems, in BPELWS instances are not created via a factory pattern. Instead, instances in BPEL4WS are created implicitly when messages arrive for the service. That is, instances are identified not by an explicit "instance ID" concept, but by some key fields within data messages. For example, if the process represents an order fulfillment system, the invoice number could be the "key" field to identify the specific instance involved with the interaction. Thus, if a matching instance isn't available when a message arrives at a "startable" point in the process, a new instance is automatically created and associated with the key data found in the message. Messages can only be accepted at non-startable points in a process after a suitable instance has been located; that is, in these cases the messages are in fact always delivered to specific instances. In BPEL4WS, the process of finding a suitable instance or creating one if necessary is called message correlation."