Some messaging transactions depend upon the exact sequence of messages from a queue, and for that sequence to be maintained in the event of a failure of the queue manager. In these instances you must serialize the access to those messages.
Serialization of messages is achieved through the use of specialized connection options, and a unique connection token when the application that empties the messages from a queue issues a connect call to the WebSphere MQ queue manager that owns that queue.
A typical situation in which WebSphere Message Broker can exploit this feature is the case where multiple brokers, with multiple execution groups, are each running message flows that empty from a shared input queue. If one broker queue manager fails, then the message flow can be automatically started on another broker while maintaining the transactional integrity and original sequencing of the messages on the shared queue.