One of the main grading systems when talking about the plywood is the smoothness of its front and back sides. This won’t imply the actual quality of wood itself, e.g. comparing the quality of luan and pecan tree, but rather the finish of surfaces. ACX and CDX are both exterior plywoods and aren’t specifying the type of wood. First letter in the abbreviation stands for the front side, the second is the reverse side which usually poorer finish. ACX has ‘A’ grade front side and ‘C’ is the grade for the other side, CDX has a bit rougher ‘C’ and ‘D’ plies. ‘A’ graded surfaces are finely sanded, easily paintable and defect-free. The use is widely spread on various furniture products. ‘B’ grade has more knots and patches, while ‘C’ is considerably worse. The knots are limited to higher diameters. Logically, the ‘D’ graded plywood will have most knots, knotholes and splits. The typical applications are decking, underlayment, floorings. Considering these facts, it is obvious that ACX plywood is higher quality, which simultaneously follows the higher price. This doesn’t mean it is stronger or more durable, since this depends on the thickness.
The letter ‘X’ on both ACX and CDX stands for exterior grade glue resin specified by plywood associations. This doesn’t mean it is waterproof necessarily, but rather resistant to moisture and weather conditions. This implies the usage in construction, and protective purposes. Building bookcases and cabinets doesn’t require exterior grade glue. When buying, always consider the actual purpose. If esthetic isn’t important, buy lower grade plywood instead of meaninglessly buying an expensive plywood which won’t be painted or affect the look. Both types can be purchased in local shops; the availability isn’t an issue as it is for exotic types of plywood.