Halliday & Matthiessen (2014: 316):
There is no very sharp line separating (circumstantial) expressions of Extent from (participant) expressions of Scope of the enhancing type; but there is a distinction between them: Extent is expressed in terms of some unit of measurement, like yards, laps, rounds, years, whereas Scope in terms other than measure units (contrast they walked five miles with they walked the streets); and being a participant, the Scope has the potential of being able to serve as Subject.