The way that it was explained to me when I started participating in the Commuting Mileage threads was that anything you rode on your way to or from work (or school, if not yet working) counted. Mid-day errands and after-work club rides counted, so long as you were not yet home from your morning ride out.

I do agree it would be kind of a linguistic stretch to say that you "commuted" to the library or grocery store if you don't work there. Does the "Utility Cycling" subforum have a thread for tracking that stuff?