Messages in a message group share the same group id, i.e., they have the same group identifier property JMSXGroupID and an integer property to identify the group sequence.
Messages in a message group are always consumed by the same consumer, even if there are many consumers on a queue. They pin all messages with the same group id to the same consumer. If that consumer is closed another consumer is chosen and will receive all messages with the same group id.
A message group can be assigned to a different consumer by having the producer send a message which has "message group signal close", which is a boolean parameter, set to true. The same parameter can be used to indicate that the consumer handling this message group can be coupled with a different message group.