Martin (1992: 474, 476):
Although it is harder to unpick the meaning of Subject in written monologue [than in spoken mode] from the meaning of Theme and Given, texts such as [6:36] above which realise their method of development through marked Themes demonstrate the significance of modal responsibility in this mode. …
Unlike Theme, Subject selection in [6:36] does not reflect the text's contrastive method of development.
The meanings of these grammatical functions are metafunctionally distinct. The meaning of Subject is interpersonal (modal responsibility) whereas the meanings of Theme (point of departure of the message) and Given (recoverable information) are textual. Because 'method of development' is a concern of the textual metafunction, Subject selection is irrelevant to a text's method of development.