Pressure-treated lumber has gone through a process that infuses a chemical into the wood. This process creates a more durable material. There will be several different types of pressure-treated lumber, and they all have different application purposes. Before you decide to purchase any, you should make yourself aware of what the stamp means so you end up with exactly what you need.
Step 1 - Find the AWPA Code
This is going to be the most important part of the stamp. This code tells you what category the wood is, and what it should be used for. UC2 will be the lowest grade of pressure-treated lumber, and this means it's protected from the weather but still subject to moisture damage. It's best used in interior construction. UC3B is best for decking, railing, and fences as it's treated to withstand prolonged moisture. UC4A is similar but can be used in applications where the wood will be underground as well, such as privacy fence posts. UC4B is best for permanent foundations and utility poles as it will resist moisture and decay if placed underground.
Step 2 - Look For Retention Level
The retention level will also give you an idea of where the lumber is best used. You may see the stamp marked with 'above ground' 'construction', or 'interior'. In most cases, this tag is placed at the top of the stamp near the year.
Step 3 - Inspection Agency
You probably aren't going to be familiar with all the inspection agencies that deal with lumber, but you should at least verify the lumber has been inspected. The mark will be either a logo of the agency or a series of initials at the left portion of the stamp before the rest of the information is found.