Halliday & Matthiessen (1999: 398):
This second–order enabling nature of the textual metafunction is seen both at the level of context, where mode (the functions assigned to language in the situation) is second–order in relation to field and tenor (the ongoing social processes and interactant rôles), and the level of content — the semantics and the lexicogrammar, where the systems of theme and information, and the various types of cohesion, are second–order in relation to ideational and interpersonal systems of transitivity, mood, and the rest.