Maybe easiest to explain with an example. I've got a watch that needs a repair. Okay, so next action is to take it to the store. Context is "@errands". But my "natural" errands take me to the grocery store, not the watch store. So unless I make a specific effort to go out to the watch store, it won't happen. On the other hand, it's not something that must happen at a specific time, so I'm not sure if I'm supposed to schedule it in my calendar, because the message I got from the GTD book was that we should be quite strict about what deserves a spot in the calendar, i.e., things that absolutely must happen on the specific date.