Halliday & Matthiessen (2014: 361):
… the English clause is a composite affair, a combination of three different structures, deriving from distinct functional components. These components (called ‘metafunctions’ in systemic theory) are the ideational (clause as representation), the interpersonal (clause as exchange) and the textual (clause as message). What this means is that the three structures serve to express three largely independent sets of semantic choice.