That’s some dense jargon. Evidently it’s from cognitive science, computer science and programing have similar phrases.
Hutchins uses the example of a Navy nav team which I am familiar with (CG) so I was able to puzzle out the meaning from that.
The phrase “the map is not the terrain” and the term “mental model” both are example of representations of reality.
"Propagation across external media just refers to the processes we use to transfer information from the real world, for example radar ranges and bearing onto another media, for example to a chart.
ECDIS has changed navigation of course but there are other examples of bridge crews processing information . For example the methods used to handle weather information on the El Faro