Final answer:
Pressure-treated lumber is rated based on preservative retention levels, which vary by the wood's intended application rather than a fixed grade system. Lumber's categorization by retention levels corresponds to how much preservative is held in the wood, which is tied to where and how the lumber will be used.
Step-by-step explanation:
The grades of pressure-treated lumber are actually not as simple as a numerical count because there are various standards and categories depending upon the use and type of preservative treatment. However, the choice of A, B, C, D presented in the question typically relates to the retention level of the preservative in the wood. Preservative retention refers to how much preservative is retained in the wood after the treatment process, and it's measured in pounds per cubic foot (pcf).
There are different retention levels specified for different applications, such as Above Ground, Ground Contact, Freshwater, and Heavy Duty Saltwater. For example, lumber that's meant to be used in ground contact situations will have a higher preservative retention requirement than lumber intended for above-ground use. In terms of actual 'grades', lumber is usually differentiated by its intended use rather than a simple grading system.
In conclusion, the 'grades' of pressure-treated lumber are based on the preservative retention suitable for the lumber's intended application rather than a clear-cut tier system. Therefore, it is not accurate to specify a particular number of grades like three, four, five, or six without understanding the context of the usage and the standards set by the American Wood Protection Association (AWPA) or similar governing bodies.