Halliday & Matthiessen (2014: 549):
Such projections may be embedded as they stand, as nominalisations – equivalent to functioning as Head. But frequently they occur as Postmodifier to a noun of the ‘fact’ class, e.g. the fact that their team had won. Fact nouns include ‘cases’, ‘chances’ and ‘proofs’, related to propositions; and ‘needs’, related to proposals. We refer to these projections, therefore, as facts. Whereas any clause that is projected by another clause, verbal or mental, is either a quote (paratactic) or a report (hypotactic, or embedded if the process is a noun), any clause that has the status ‘projected’ but without any projecting process is a fact and is embedded, either as a nominalisation serving as Head or as Postmodifier to a ‘fact’ noun serving as Head. This includes some of those functioning in mental clauses, as mentioned above, and all projections functioning in relational clauses (since a relational process cannot project). It also includes ‘impersonal’ projections such as it is said ... , it is believed ... , it seems ... , where the ‘process’ is not really a process at all, but simply a way of turning a fact into a clause.