Description: Represents the value contained in MSH 10.1 (the message control id).
Programming Note: The Message Control ID field of the MSH segment is special. The worldwide HL7 standards state that when used the message control ID field should represent an absolutely unique identifying string value to identify this particular HL7 message. The value does not have to be (and usually isn't) globally unique (ie a GUID, in fact in HL7 standards this field is too small to contain a GUID) but should be unique between the sender and the receiver.
Following this logic. If the sending party is adhering to HL7 standards then the concatenated values of: Sending Application, Sending Facility, Receiving Application, Receiving Facility, Message Type, Message Event AND Message Control ID should represent a guaranteed unique value identifying a particular message.