Halliday & Matthiessen (1999: 170-1):
… participants are located at some point along a scale of individuation, ranging from the most generalised (e.g. diamonds are forever) to the most individuated (e.g. Elizabeth’s diamonds were stolen). The Range element tends towards the generalised end of the scale. This is especially the case with those of the elaborating type, where the Range usually represents a general class; and it is always the case if the figure is one of being, with Range as Attribute. For example:Peter plays tennis (cf. is a tennis-player)Peter plays the piano (cf. is a pianist)His opinion is not important (cf. does not matter)