For each license code, you can use it to validation your staging site and your production site at the same time.
if ever you have converted the license from X to Pro, you will have two license. One is the X license and the other one for the upgraded Pro theme license. When it is in red color, it indicates that the license is unlock and can be transferred to other account. You must lock the license at all times.
When it is locked, it cannot be transferred or used to register a new APEX account.
Hope this helps.