Activation codes are generally invalid once used, and they are not irreversible once activated. There are two main types of activation codes: one is the activation code for general software, and the other is the activation code for games. Some software will add password programs and use activation codes in order to make their software face a specific paying crowd. Only by entering the correct password can you use this software normally. It's like locking a door with a lock. Only people with keys can open the door.