Once you've completed the Rent To Own program for your Circle2 plugin, Splice will email you a copy of the serial number to the email associated with your Splice account. Your Circle2 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.