Halliday & Matthiessen (1999: 205):
Since things of these various kinds are the names of semantic relationships, it is not surprising that their status is somewhat ambivalent. Consider an expression such as a volume of poetry: this may be volume as Thing, qualified as being concerned with poetry, e.g. she picked up an old volume of poetry from the shelf, or Facet (extending: amount), e.g. she has just published a new volume of poetry. The latter example shows up the nature of the ambivalence: the quality 'new' is presumably a quality of the poetry (cf. I'd like a strong cup of tea).
Similarly we have the ambivalent relationship between parts and wholes (the basis of synecdoche), and between symbols and what they represent, regarding which to construe as the participant in particular figure. In general, the uncertainty that arises is whether the expansion is to be construed as an independent thing or as a facet of something other than itself. This uncertainty is sometimes foregrounded under pressure from the textual metafunction: as Theme, for example, do we say the end of that story you're never going to hear! or that story you're never going to hear the end of?