EMA C++ Performace Tools Guide : 10 Performance Best Practices : 10.2 Enterprise Message API Best Practices : 10.2.2 submit
 
10.2.2 submit
To make efficient use of underlying transport function calls, the submit function passes messages to an outbound queue of the specified priority, rather than immediately writing the message to the network.
The network write occurs if:
Enterprise Message API internally calls rsslFlush on the Channel instance in the Transport layer.
HighWaterMark configuration parameter in Channel or Server group submit If DirectWrite is set, the API will flush each message upon submission without waiting to queue data. If set in conjunction with highWaterMark, directWrite takes precedence.