Halliday & Matthiessen (2014: 170):
So far we have been concerned with ‘free’ clauses; the term ‘free’ is the entry condition to the system of MOOD TYPE. Semantically, this means that ‘free’ clauses realise either propositions or proposals, serving to develop exchanges in dialogue either by initiating new exchanges or by responding to ones that have already been initiated. In contrast, ‘bound’ clauses are not presented by the speaker as being open for negotiation.