What's the difference between fastening torque and breakaway torque?

Fastening (or 'nut-tightening') torque is how hard the tool can drive a fastener on. Breakaway (or 'nut-busting') torque is how hard it can crack a stuck fastener loose, and is usually the higher of the two numbers. Marketing copy almost always leads with breakaway torque because it's the bigger figure, so compare like-to-like when shopping.