A module's threads may act as either initiators or targets. An initiator is responsible for creating a payload (see Section 2.3.1) and calling the transport function to send it. A target receives payloads from the transport function for processing and response. In the case of non-blocking interfaces (see Section 2.3.3), the target may create new transactions backwards in response to a transaction from an initiator.

Initiator calls are made through initiator sockets, target calls received through target sockets. A module may implement both target and initiator sockets, allowing its threads to both generate and receive traffic.