Halliday & Matthiessen (2014: 48):
Another interesting issue is to what extent different semiotic systems extend all the way along the cline of instantiation from the instance pole to the potential pole. We can ask of any one given semiotic system how systemic it is — which clearly relates to the question of how much individual variation there is across a speech fellowship (or speech community). … it is theoretically quite possible that certain other semiotic systems are more usefully interpreted as operating with systems located somewhere midway along the cline of instantiation; in other words, they are most usefully described in register-specific terms. For example, if we consider semiotic systems that have been included under the heading of ‘visual semiotics’, we can note how highly contextually adapted and specialised systems such as technical drawing, mass transport route cartography and press photography are; it is not immediately clear that they can all be regarded as registerial subsystems of a general visual semiotic system.