You free up an activation slot when you deactivate it. You don't recieve any codes. I believe you can have it activated up to 3 computers at once. So lets say you have it activated on one computer right now, you can activate it on 2 more computers. Now if you uninstall it and deactivate it on your one computer, now you can activate it on 3 computers once again, because its now deactivated on said computer.
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