Halliday & Matthiessen (2014: 114-5):
Below the clause complex, the grammar manages the discourse flow by structural means; and here there are two related systems at work. One is a system of the clause, viz. THEME; this we have been discussing throughout the present chapter so far. The THEME system construes the clause in the guise of a message, made up of Theme + Rheme. The other is the system of INFORMATION.