Halliday & Matthiessen (1999: 203):
Things of this type fall into four subcategories, defined by their interaction with modality: cases, chances, proofs and needs. Cases represent simple facts; chances represent facts to which some degree of modalisation is attached; proofs are demonstrations of facts [caused modalisation]. The fourth category, that of needs, are facts accompanied by modulation, that is, where the projection is that of a proposal rather than a proposition.