Halliday & Matthiessen (2014: 206):
Unlike the Theme, which — while it is itself a property of the clause — carries forward the development of the text as a whole, the Mood element has little significance beyond the immediate sequence of clauses in which it occurs. It tends to be the overall organisation of the text that determines the choice of Theme in any particular clause, or that determines at least the general pattern of thematic choices; whereas there may be no general pattern in the choice of Subject, but only a specific propositional basis for each exchange. … Nevertheless, the ongoing selection of Subjects by a speaker or writer does give a characteristic flavour to a piece of discourse.