Logical message
A logical message is a single unit of application information. In the absence of system constraints, a logical message is the same as a physical message. Logical messages can be extremely large; system constraints might make it advisable or necessary to split a logical message into two or more physical messages known as segments.
A logical message that has been segmented consists of two or more physical messages that have the same non-null group identifier (GroupId field in MQMD) and the same message sequence number (MsgSeqNumber field in MQMD). The segments are distinguished by differing values for the segment offset (Offset field in MQMD), which gives the offset of the data in the physical message from the start of the data in the logical message.