What is the difference between rubber timing belts and rubber flat belts?

What is the difference between rubber timing belts and rubber flat belts?

Rubber Timing Belts: Have teeth on the inner circumference that mesh with pulley grooves to provide positive, non-slip drive with precise speed ratios. The backside may be flat or textured.
Rubber Flat Belts: Have a smooth or coated surface and rely on friction for power transmission. They allow for some slip, are suitable for long center distances or variable speed applications, but cannot guarantee synchronous motion.