I am assuming you are referring to the Class A surface designation found within the automotive industry.
A Class A surface is any surface found on a vehicle that immediately presents itself to casual visual inspection by a discerning customer. This would include and is not limited to seats, bumpers, steering wheels and dash covers.
Class A surface is used (usually by one of the big guys) when the standard processing of the particular component can render a product to have a visual discontinuity which could be perceived as a defect. This could be something as simple as the chrome isn't shiny enough to variations in paint finish which create optical illusions of deformed surfaces.
What usually happens is that early on in the product development a visual standard for the product is established using sample defective and good pieces. In some cases the standard be purely subjective or in others it can be dependant on (for example) a measurement of light refraction.
