A staggered fitment is also referred to as a staggered application. Basically, it means that the wheels on the back of the vehicle are a different size than those on the front. Wider wheels are usually run on the back, for example a 19x8 on the front and 19x9.5 on the rear. This is typically done on rear wheel drive cars such as Infiniti G35, Nissan 350Z, BMW, Mercedes, Audi, VW, Ford Mustang, and others. Having staggered wheels or running wider wheels on the drive wheels means more grip on acceleration and from an aesthetic perspective, larger lips.
A staggered fitment can also mean larger diameter wheels on rear, for example a 17" diameter on the front and 18" diameter on rear. Some examples of this would be aftermarket Acura NSX and Chevy Corvette. TSW manufactures the largest range of staggered wheels.
Wheel Terminology Explained