Halliday & Matthiessen (2014: 111-2):
(i) Initial position in the English clause is meaningful in the construction of the clause as message; specifically, it has a thematic function.
(ii) Certain textual elements that orient the clause within the discourse, rhetorically and logically, are inherently thematic.
(iii) Certain other elements, textual and interpersonal, that set up a semantic relation with what precedes, or express the speaker’s angle or intended listener, are characteristically thematic; this includes finite operators, which signal one type of question.
(iv) These inherently and characteristically thematic elements lie outside the experiential structure of the clause; they have no status as participant, circumstance or process.
(v) Until one of these latter appears, the clause lacks an anchorage in the realm of experience; and this is what completes the thematic grounding of the message.