Appearance stamps on plywood range from cabinet-grade (A) to construction grade (C and D). Exterior plywood carries an X stamp, which identifies it as suitable for use outdoors. Beside CDX, you can also find better grades, such as ABX, ACX and even BCX.
When purchasing exterior plywood, one of the first things the home owners look for is grade. Like other types, exterior plywood is classified based on appearance using letter grading system, A through D, with each letter representing the quality of the wood.