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In radio communications, the message part or parts that (a) precede the text, i.e., the message body, in time or space according to established conventions and (b) may include several data items, such as address groups, routing indicators, action addressee designators, information addressee designators, exempted addressee designators, prosigns, prowords, clear indicators, date-time groups, originator designators, special instructions, and protocol symbols. Note: Several message heading data items may be combined into a message preamble. [From Weik '89]