Final answer:
The "X" in CDX plywood stands for exterior use.
Step-by-step explanation:
The "X" in the plywood grade CDX stands for eXterior.
CDX is a common grade of plywood used for construction purposes. The "C" represents that the wood on one side of the plywood is of a lower grade, while the "D" indicates that the wood on the other side is even lower in quality. The "X" signifies that the plywood is designed for exterior use, meaning it is suitable for applications exposed to the weather, such as roofs and subfloors.