Halliday & Matthiessen (2014: 218-9):
Part of the ‘flavour’ of a particular text, and also of the register that it belongs to, lies in its mixture of process types. For example, in enabling contexts, recipes and other procedural texts are almost entirely ‘material’, whereas, in reporting contexts, ‘verbal’ clauses play an important role in news reports and, in sharing contexts, ‘mental’ clauses are a typical motif in casual conversation. The mixture of process types characteristic of a text belonging to a particular register typically changes in the course of unfolding of the text.