Halliday & Matthiessen (2014: 471):
In extension, one clause extends the meaning of another by adding something new to it. What is added may be just an addition, or else a replacement, or an alternative. There is a closer parallel with extension between parataxis and hypotaxis than we find with elaboration; we can operate with a single system of categories for both kinds of taxis, although there are certain gaps in the paradigm (e.g. negative additive relations are only paratactic, not hypotactic).