Halliday & Matthiessen (1999: 89):
The ideation base construes phenomena as organic wholes that may take on rôles in other kinds of phenomena; but it also deconstrues many such organic wholes into their component parts. When these component parts are phenomena of the same type […] this is known as meronymy (or meronymic taxonomy; cf meros ‘part’). We find local taxonomies of this kind, often interlocked with hyponymies. Taxonomies thus embody the two types of expanding relationships […] extension and elaboration. The meronymic type of taxonomy is extension, whereas the hyponymic type is elaboration.