Halliday & Matthiessen (1999: 506):
The system is polysystemic from the point of view of contextual diversification: given a particular context type or a set of related types, some parts of the overall semantic system will be deployed in a systematic way whereas others will remain inactive.
For instance, if contexts where the goal is to instruct somebody in a procedure were the only source of evidence for the semantic system, we could safely ignore figures of sensing and saying and the logical relationship of projection.
And yet in the general system, which is the overall meaning-making resource deployed across all the different types of context, these figures and sequences turn out to be essential both to the functioning of the semantic system and to our own understanding of its fundamental principles.