Halliday & Matthiessen (1999: 156):
The manner of performance of a process may vary, in which case it is the Medium by which it is typically determined. This may be a major variation in the mode of actualisation, for example ‘open + door, open + account, open + eye’ where the process is respectively mechanical, verbal, or physiological; or simply a minor difference in the means that is employed, e.g. ‘brush + teeth, brush + clothes’. In many cases, the difference in the manner of performance is the basis of a lexically codified taxonomic distinction.