Every computer has an associated chipset and bios. These are used to generate a unique ten-digit code that identifies the user computer.
You can find out your computer 10-digit code by clicking here.
When each eBook is opened, a field is available to enter the ten-digit code. Once the code is entered the eBook is fully functional. The software remains fully functional until the user clears the computer cache. In some cases the cache does not clear the code from the software in which case you do not need to re-enter.