Once you've completed the Rent-To-Own program for your Circle 2 plugin, Splice will email you a copy of the serial number to the email associated with your Splice account. Your Circle 2 plugins will continue to run with the Splice app open.
If you want to use your plugins without the Splice app, you can register your serial number by:
Launch the Circle2 plugin and click on the FAW logo at the top left:
You will be able to enter your lifetime serial number and use Circle2 offline without Splice.