It is now widely or even universally accepted that computer systems that are used within healthcare to record information about the care given to patient's need to share that information with other computer systems and their users. In order that computer systems may share information effectively there is a requirement that the communicating parties, and particularly their computer systems have a common understanding of how the information which they are sharing is represented.This sharing of representation needs to take place at a number of levels, most notably at the data representation or syntactic level which is the subject of CEN/TS 14796, but also at the macro or semantic level where groupings of data are used to provide a context or set of contexts for the data. This Part 4 of the standard is limited to descriptions of components concerned with messaging, and in particular the message and batch headers.