General purpose receptacles are any receptacles other than kitchen countertop receptacles, kitchen island receptacles or receptacles supplying a single fixed-in-place appliance, such as a dishwasher, garbage disposal unit, trash compactor, microwave oven or refrigerator. The NEC has nothing to say about the mounting height of general purpose receptacles, but the accepted convention in the trades is that they be mounted so the top of the receptacle box is 14 inches above the finished floor line.
The NEC gives specific instructions about the mounting height of kitchen counter receptacles. Although by convention they are mounted so the bottom of the box is 4 inches above the countertop’s backsplash, they may be mounted as high as 20 inches above the countertop. They may also be mounted up to 6 inches below the countertop and over cabinet doors and drawers, as long as the countertop’s overhang does not extend 6 inches or more beyond the front of the cabinets.
NEC rules governing the placement of receptacles to kitchen islands are the same rules that apply to general purpose receptacles. The basic idea is to have enough receptacles conveniently installed to eliminate the need to use extension cords. Extension cords not only increase the chances of overloading an electrical circuit, but they also create tripping hazards.
Special purpose receptacles -- single receptacles placed on dedicated branch circuits serving fixed-in-place appliances -- can be mounted at any height that's convenient for the appliance being served, which is usually directly behind the appliance. Some specialized receptacles, such as clock hanger receptacles, have a mounting height determined by where you want the clock to be located. Clock hanger receptacles can be connected into any general purpose receptacle branch circuit. The placement of receptacles serving fixed-in-place appliances is primarily determined by the appliance.