Halliday & Matthiessen (2004: 207):
… this means that it is less frequent and that it carries a special interpretation.
Obviously clauses construing doing and clauses construing sensing are different in meaning, but that is not enough to make them constitute distinct grammatical categories. There are indefinitely many ways of drawing lines on purely semantic grounds, … but the question we are concerned with here is which of these have systematic repercussions in the grammar.
… grammatical labels are very rarely appropriate for all instances of a category — they are chosen to reflect its central or ‘core’ signification ( … ‘prototypes’ …). These core areas are the central region for each process type … and the non-core areas lie on the borders between the different process types, shading into one another as the colours of a colour spectrum.