Halliday & Matthiessen (2014: 41):
Orientation towards field means that the goals of the situation, or intended outcomes, are concerned with field, more specifically with the development of field, as in an ‘expounding’ context where the speaker’s goal might be to construe a taxonomy for the addressee, a classification of some classes of phenomena. When texts operate in such situations, they tend to be organised in terms of field — in terms of the structure of the field, as when a text is organised according to the classes of a taxonomy. Orientation towards field is characteristic of ‘expounding’, ‘reporting’ and ‘doing’ contexts, and also in principle of ‘enabling’ contexts of the ‘instructing’ subtype.