flow control restriction

No definition is given in NIST SP 800-171r3. The term is attested in use at 1 citation in that document; a definition is pending curation.

The set of specific, policy-derived enforcement mechanisms that constrain how information may move across boundaries within or between systems — governing the *path and channel* of data rather than who is authorized to read it. Distinguished from access control by its focus on transmission routes, content characteristics, and inter-system transfer conditions (e.g., blocking spoofed traffic, preventing cleartext export of controlled data, limiting proxy bypass), these restrictions operationalize an organization's information-flow control policy as enumerable, enforceable rules applied at network, application, and organizational boundaries. In practice, as articulated in NIST SP 800-53 AC-4, the concept governs where information can travel within and between systems — distinct from who is allowed to access it — with concrete examples such as blocking external traffic claiming an internal origin, keeping export-controlled information from being transmitted in the clear, and limiting transfers based on data structures and content.