Halliday & Matthiessen (2014: 76):
We shall also refer to three classes of group: verbal group, nominal group, adverbial group (also preposition group and conjunction group); and to one class of phrase: prepositional phrase. … We shall not need to discuss clause classes explicitly, although they are in fact present as part of the overall description, as in the distinction between major and minor clauses and within major clauses between free and bound clauses.