As a general rule of thumb, scrambling starts at the point where you need to start using your hands to make progress or feel secure. A Grade 1 scramble won’t require a rope - unless of course it’s winter, when it might be ice-glazed or quilted in deep snow. It may be steep but not too exposed and the steps up should be firm and obvious. Or, on the other hand, it may be very exposed (like Crib Goch) but not steep at all. Grade 2 routes will mix exposure and steepness more freely, be harder to backtrack on or abort, and the footholds and handholds may be smaller and further apart.