Halliday & Matthiessen (1999: 92):
While the choice of 'level' in hyponymic elaboration is the choice in delicacy of categorisation, the choice of level in a meronymic taxonomy is the choice in delicacy of focus. The focus is typically on the whole (i.e., the most inclusive region within the meronymy) even if a specific part is particularly important (cf. Langacker, 1984).