Halliday & Matthiessen (2004: 428):
The meaning of an embedded clause, or phrase, that is functioning as an expansion is essentially to define, delimit or specify. Thus the characteristic embedded expansion is the ‘defining relative clause’ … . Its function is to specify which member or members of the class designated by the Head noun … is or are being referred to. … The typical defining relative clause, introduced by who, which, that, or in its so-called ‘contact clause’ form without any relative marker … , is elaborating in sense.