Halliday & Matthiessen (2014: 548):
Both verbal and mental acts have names, such as statement, query, belief, doubt; and these also serve to project, with the projected clause embedded as Postmodifier: the belief that the sky might fall on their heads. There is a point of overlap between these and embedded expansions of the elaborating type (relative clauses): both may be introduced by that, and this produces ambiguities such as the report that he had submitted disturbed everyone:(a) the report [[ = that he had submitted ]]
‘the document which he had drafted’
(b) the report [[ “Ithat he had submitted ]]
‘to hear that he had yielded’