Halliday & Matthiessen (2014: 549-50):
The sequence of clauses within a clause complex is textually significant from a thematic point of view. This sequence is fixed in the case of parataxis (with the exception of parataxis in the environment of projection); here the primary clause is in a sense thematic in relation to the secondary one even though the sequence is fixed: the primary clause may serve as a point of departure or orientation for the secondary clause. However, in the case of hypotaxis, the sequence is not fixed; it can be progressive (α ^ β) or regressive (β ^ α) – or the hypotactically dependent clause may be included within the dominant clause (α << β >>). For example, the hypotactic nexus β as he came to a thicket, α he heard the faint rustling of leaves is regressive, with the dependent clause as he came to a thicket given thematic status within the nexus. In hypotactic nexuses of certain logicosemantic types, the dependent clause may be the focus of theme predication.