Halliday & Matthiessen (2014: 493):
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’ (also called ‘restrictive’), like that Jack built in the house that Jack built. Its function is to specify which member or members of the class designated by the Head noun, in this instance house, is or are being referred to.