Halliday & Matthiessen (2004: 470-1):
There are four subclasses of fact noun: (1) cases, (2) chances, (3) proofs and (4) needs. The first three go with embedded propositions whereas the last goes with embedded proposals … . The first three differ in terms of the modality of the subtype of modalisation:
(1) cases (nouns of simple fact) relate to ordinary non-modalised propositions ‘it is the case that …’
(2) chances (nouns of modality) relate to modalised propositions ‘it may be (the case) that …’(3) proofs (nouns of indication) relate to propositions with indications, which are equivalent to caused modalities, ‘this proves/implies (ie makes it certain/probable) that …’