Halliday & Matthiessen (2014: 494):
The typical defining relative clause, introduced by who, which, that, or in its so-called ‘contact clause’ form without any relative marker (e.g. he told in the tales he told), is elaborating in sense. The following example illustrates the contrast between an embedded, defining relative clause and a hypotactically dependent, non-defining one.
||| The only person [[ who was kind to him at all]] was the Skin Horse, || who had lived longer in the nursery than any of the others. |||
The relative element in an embedded clause restates the nominal antecedent; thus in
the man [[ who came to dinner ]] stayed for a month
the man who came to dinner and the man who stayed for a month are the same man. This is the same principle by which non-defining relatives are also elaborating in function. The defining ones, however, do not form a separate tone group, because there is only one piece of information here, not two – who came to dinner is not news, but simply part of the characterisation of that particular participant.