Maximum torque is the 'peak' amount of torque the tool outputs assuming the bolt is attached to a solid material. If a bolt is attached to something like a spring, then the force (torque) applied gets absorbed and is reduced via the laws of physics. There are many variables affecting how torque behaves in the real world. Wikipedia has a great explanation on the variables of torque. "Nut-Busting" torque has NO agreed upon definition, but generally speaking, it refers to the capability of a tool (IE. impact wrench) to remove a bolt that has been tightened via a method other than with the tool being tested. In other words, if a bolt is tightened with a calibrated wrench at 1000 ft./lbs. and the impact wrench being tested is able to remove the bolt immediately after it has just been tightened --- that becomes the measure (number) for "Nut-busting torque" assigned and marketed. But, this is misleading, because it's far easier to remove a freshly tightened bolt as opposed to a rusted bolt on an old car. It sounds good though. The higher the number, the better in the eyes of unassuming consumers. Read more about torque testing direct from Ingersoll Rand.