Halliday & Matthiessen (2014: 172):
‘Bound’ clauses are, as we have suggested, presented as presumed rather than as negotiable. They lack a number of the interactive features of ‘free’ clauses. They are very unlikely to be tagged even if they are ‘finite’ and thus look structurally like ‘declarative’ clauses. … And ‘non-finite’ clauses cannot be tagged. Similarly, Vocatives and speech-functional comment Adjuncts – both highly interactive features – are unlike to occur with ‘bound’ clauses.