Halliday & Matthiessen (2004: 468-9, 470):
Nouns that project belong to clearly belong to clearly defined classes, verbal process nouns (locutions) and mental process nouns (ideas); they correspond rather closely to, and in many instances are derived from, the verbs used in the projecting clause, especially the reporting ones. …
… 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 (eg the thought [[that she might one day be queen]]) or by expansion (eg the thought [[that came into her mind]]). This leads to ambiguities such as the report [[that he was submitting]] …