Codes are a form of referral link, your referent is getting some reward for your successful verification in the network, it's one time reward. You can't mine on multiple machines, or at least it would be extremely difficult to register yourself as a real person on more than one machine, because of how the network is protected. Once per several days, at the exact hour, captcha puzzles appear for you to solve within specific time frame. It would be very hard to complete those puzzles on more than 2 clients. Bad answers are also penalized, so you are at risk of loosing verification rewards and even an entire wallet.