Message group
A message group is a set of one or more logical messages that have the same non-null group identifier. The logical messages in the message group are distinguishable by differing values for the message sequence number. The message sequence number is an integer in the range 1 to n, where n is the number of logical messages in the group. If one or more of the logical messages is segmented, the group will contain more than n physical messages in the group.