Messages consist of lines of text. No special provisions are made for encoding drawings, facsimile, speech, or structured text. No significant consideration has been given to questions of data compression or to transmission and storage efficiency, and the standard tends to be free with the number of bits consumed. For example, field names are specified as free text, rather than special terse codes.
A general "memo" framework is used. That is, a message consists of some information in a rigid format, followed by the main part of the message, with a format that is not specified in this document. The syntax of several fields of the rigidly-formated ("headers") section is defined in this specification; some of these fields must be included in all messages.
The syntax that distinguishes between header fields is specified separately from the internal syntax for particular fields. This separation is intended to allow simple parsers to operate on the general structure of messages, without concern for the detailed structure of individual header fields. Appendix B is provided to facilitate construction of these parsers.
In addition to the fields specified in this document, it is expected that other fields will gain common use. As necessary, the specifications for these "extension-fields" will be published through the same mechanism used to publish this document. Users may also wish to extend the set of fields that they use privately. Such "user-defined fields" are permitted.
The framework severely constrains document tone and appearance and is primarily useful for most intra-organization communications and well-structured inter-organization communication. It also can be used for some types of inter-process communication, such as simple file transfer and remote job entry. A more robust framework might allow for multi-font, multi-color, multi-dimension encoding of information. A less robust one, as is present in most single-machine message systems, would more severely constrain the ability to add fields and the decision to include specific fields. In contrast with paper-based communication, it is interesting to note that the RECEIVER of a message can exercise an extraordinary amount of control over the message's appearance. The amount of actual control available to message receivers is contingent upon the capabilities of their individual message systems.