Halliday & Matthiessen (2014: 35):
Field, tenor and mode variables are the basis for any attempt to develop a taxonomy of situations. At the same time, since text is language functioning in context, the field, tenor and mode variables are also the basis of any attempt to develop a taxonomy of texts operating in situations. It is certainly true that in developing a taxonomy of texts, we can adopt – we need to adopt – a trinocular perspective, matching up contextual, semantic and lexicogrammatical considerations to support the taxonomy. However, to be meaningful, a taxonomy of texts must be grounded in contextual considerations. If the taxonomy is ‘on the right track’, semantic and lexicogrammatical considerations will align themselves with the contextual ones.