The combination of elaboration with hypotaxis gives the category of NON-DEFINING RELATIVE CLAUSE (also called ʻnon-restrictiveʼ, ʻdescriptiveʼ) This functions as a kind of descriptive gloss to the primary clause, as in They decided to cancel the show, which upset everybody alike. These dependent clauses may be either finite or non-finite [...] If the secondary clause is finite, it has the same form as a defining relative clause of the WH-type. It differs from a defining relative clause, however, in two ways: there is a distinction in the meaning, and there is a corresponding distinction in the expression, both in speech and in writing. As far as the meaning is concerned, these clauses do not define subset, in the way that a defining relative clause does. In 'the only plan which might have succeeded' the defining clause 'which might have succeeded' specifies a particular subset of the general class of plans. A non-defining relative clause, on the other hand, adds a further characterization of something that is taken to be already fully specific. This ʻsomethingʼ, therefore, is not necessarily just a noun; the domain of a non-defining relative may be a whole clause [...] or any of its constituents.
- Halliday (1985), a pag.204