Halliday & Matthiessen (2014: 474):
The combination of extension with hypotaxis also embraces (a) addition, (b) variation and (c) alternation, but with the extending clause dependent. The dependent clause may be finite or non-finite. Compared with paratactic extension, the hypotactic type appears to be fairly rare; it is, in fact, the least common of the combinations of types of expansion and types of taxis.