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 roles), and at the levels 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.