A license is for use on one computer and is activated using a code (Request number) unique to that computer.
Because many people use a desktop and a laptop, you are given a serial number that can be activated twice. This means you can use the software on two computers.
If you want to use it on a third you need to either...
1) Buy another serial number
2) Release the license on one of your previously installed computers.
However, you can't move a license between two computers by repeatedly Releasing and Re-activating. Release can only be done a few times.