Halliday & Matthiessen (2014: 535):
… the noun is the name of a locution or an idea, and the clause that it projects serves to define it in exactly the same way that a ‘restrictive’ relative clause defines the noun that is expanded by it. Hence any noun that belongs to a projecting class may be defined (restricted) in either of these two ways, either by projection (e.g. the thought [[that she might one day be a queen]]) or by expansion (e.g. the thought [[that came into her mind]]). This leads to ambiguities such as the report [[that he was submitting]].