Requirement. Something that, between agents, must be, but which those agents nevertheless expect from the start may not necessarily be, including for instance: imperatives in text; images that warn; spaces that confine; objects whose essential use is framed by their function; gestures that command; warning sounds; authoritative tones of voice.

Reference: Cope, Bill and Mary Kalantzis, 2020, Making Sense: Reference, Agency and Structure in a Grammar of Multimodal Meaning, Cambridge UK: Cambridge University Press, p. 251.