Halliday & Matthiessen (2014: 426-7):
… the mapping between classes at group/phrase rank and functions at clause rank is fairly complex: a group/phrase of a given class can typically serve a number of different clause functions (the exception being the verbal group). When we move down one step along the rank scale to consider the relation between word classes and group/phrase functions, we find there is a stronger tendency towards a one-to-one relationship: a word of a particular class tends to serve only one group/phrase function: see Table 6-19. The major exception is the class of adverb; but this is partly a matter of delicacy: certain adverbs function only as Head, whereas others function only as Modifier or Sub-Modifier.
