As per EMS Specification:

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JMSXGroupID and JMSXGroupSeq are defined as standard properties by the JMS specification and are provided by the following methods: 

set: 

message.setStringProperty("JMSXGroupID", "My Group");

message.setIntProperty("JMSXGroupSeq", 12345);

get:

String strValue = message.getStringProperty("JMSXGroupID");

int iValue = message.getIntProperty("JMSXGroupSeq");

These two properties, JMSXGroupID and JMSXGroupSeq, are used by client applications for grouping messaging. They work as two variables for applications to attach group information. The grouping is at the application level which is transparent to the EMS server as there is no server logic for filtering messages by groups. The EMS server does not address sending all messages having the same JMSXGroupID to only one consumer, rather the consumer application should have built-in logic to do so by using the message selector feature provided with EMS. 

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So EMS server will not group the messages and neither it will sequence the messages.It is left to JMS clients how to use these properties.