Halliday & Matthiessen (2014: 465):
If the secondary clause is finite, it has the same form as a defining relative clause of the WH- type, which is embedded as a Qualifier in a nominal group. 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 subsets, 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 characterisation of something that is already taken to be fully specific. This ‘something’, therefore, is not necessarily just a noun; the domain of a non-defining relative may be a whole clause, or any of its constituents.