Halliday & Matthiessen (2004: 482):
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 of ‘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 … 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.