message content

The substantive payload of a communications unit — the data body conveying the actual information being sent — as opposed to routing and control metadata carried in headers. In information-flow enforcement contexts (NIST SP 800-53 AC-4, SP 800-171 control 03.01.03, CMMC AC.L2-3.1.3), it serves as the basis for deep inspection by boundary protection devices — gateways, firewalls, guards — that must look beyond header attributes (source/destination, port, protocol) to examine what a message actually says or contains, using techniques such as keyword searches or document-characteristic analysis, in order to make allow/deny decisions. Its defining role in security and compliance is thus to mark the boundary between shallow, metadata-based packet filtering and content-aware, payload-level filtering for information flow control, data loss prevention, and cross-domain security enforcement.