When you run the first process in a locked application, Appx acquires a USAGE (this means that you are using 1 user right from that product registration). When you run the second, third, fourth, etc. processes in that application, you don't count as 2, 3, 4, etc. users - just a single user.

When you end ALL processes in the locked application, it decrements the usage count for the appropriate lock. If the usage count for a lock goes to zero, the lock is forfeited, and therefore the overall user count for that lock is decremented. (You are no longer counting against the user rights for that product registration)