As the numbers increase, the protection shifts from handguns to high-velocity rifles. These levels are where the density and thickness of the materials (glass, steel, or composite) increase dramatically.
Here is an overview of the most common levels referenced in the standard: ul 752 standard pdf
Using an unverified, free PDF for a real-world security installation exposes you to massive liability. If a shooter penetrates a barrier that failed because you followed an old, incorrect test method, you are legally liable. As the numbers increase, the protection shifts from