Per AMQ 6.3 product documentation
(https://access.redhat.com/documentation/en-us/red_hat_jboss_a-mq/6.3/html-single/product_introduction/) "3.4 Reliability - Message Groups" chapter states:

"This feature enables multiple consumers on the same queue to process, in FIFO order, messages tagged with the same JMSXGroupID. It also facilitates concurrency as multiple consumers can parallel process different message groups, each identified by a unique JMSXGroupID.
....
For details, see ActiveMQ in Action (Snyder, Bosanac, and Davies). "

Subsequently, the "ActiveMQ in Action (Snyder, Bosanac, and Davies)" book's corresponding chapter 12.2 "MESSAGE GROUPS" chapter reads as:
"The ActiveMQ broker will add a sequence number to each message in a group, using the standard JMSXGroupSeq message header property. The sequence number will start from 1 for a new message group. "

However, the info in "ActiveMQ in Action" does not match the way ActiveMQ works:

1- ActiveMQ broker DOES NOT add a sequence number to each message in a group
2- For getting message ordering by one single consumer, It is important to set JMSXGroupSeq by the producer to sequent number starting from 1.

The documentation should add something like this to clarify how JMSXGroupSeq works:
" In order to implement message ordering, producer should set a unique JMSXGroupID for messages produced, as well define message sequence number by setting JMSXGroupSeq value for each message".