Halliday & Matthiessen (2014: 196):
Alarms bear some resemblance to exclamatives, if only in voice quality; but they are addressed to another party, and they are in general derivable from the grammar of the clause — they are intermediate between major and minor clauses. Alarms include (a) warnings … (b) appeals … . Many of these are clearly imperative and can be analysed as such: Residue only … . Other[s] are nominal groups …