Halliday And Matthiessen (2014: 674-5):
The different domains of manifestation of expansion, illustrated for cause in Table 10-4, are textually distinct. When the domain of manifestation is a cohesive sequence of clauses or a nexus of tactically related clauses, the conjunction group realising the causal relation is given a textual status within the clause in which it appears: it is either textual Theme or part of the Rheme. While a conjunction group with a structural conjunction such as so or because as Head is obligatorily thematic, there is a choice for conjunction groups with a cohesive conjunction such as consequently as Head …
In a hypotactic nexus, there is a further textual contrast that is not open to cohesive sequences and paratactic nexuses: the dependent β clause representing the cause may be either thematic or rhematic within the clause nexus; …
When the domain of manifestation of expansion is a simple clause, the potential textual status of the manifestation of the cause depends on how it manifested – (1) as minor Process within a prepositional phrase serving as a circumstance of Cause, (2) as Process, or (3) as Thing within a nominal group serving as a participant in a circumstantial relational clause.
(1) When it is manifested within a circumstance of Cause, the cause may be given the status of either Theme or Rheme, and, if it is Theme, it may be given the status of predicated Theme; …
(2) When the causal relation is manifested as Process (either in a hypotactic verbal group complex in a clause of any process type, or as the nuclear process in a circumstantial relational clause), its textual status will most likely be rhematic. More specifically, it is likely to be (part of) the transition between Theme and New. …
(3) When the cause is manifested as the Thing in a nominal group serving as a participant, it will have the thematic status assigned to that nominal group as a whole – either thematic or rhematic. But, in addition, it will be within the domain of operation of another textual system – the system of REFERENCE. This means that it is given a textual status as a discourse referent – either recoverable (identifiable) or non-recoverable (non-identifiable), and that it can be tracked in the development of the discourse.