You need to purchase a license in the following situations:
Your trial license expired. Symantec Endpoint Protection comes with a trial license that lets you install and evaluate the product in your environment.
Your current license is expired.
Your current license is over-deployed. Over-deployed means that you have deployed more clients than your current license allows.
Depending upon how you purchase your license, you receive by email either a product license serial number or a Symantec License file. The license file uses the file extension .slf. When you receive the license file by email, it is attached to the email as a .zip file. You must extract the .slf file from the .zip file.
Save the license file to a computer that can be accessed from the Symantec Endpoint Protection Manager console. Many users save the license on the computer that hosts Symantec Endpoint Protection Manager. Many users also save a copy of the license to a different computer or removable storage media for safekeeping.
To prevent corruption of the license file, do not open or alter the file contents in any way. However, you may copy and store the license as desired.