Well, they handle it in one of two different ways.

When you buy a multi lock at a locksmith, they either issue you a card (like a credit card) with YOUR key bittings ('cuts' or 'combination' of THAT key), OR they file the card securely and set up a key authorization sheet and file that as well. Your choice. If you choose to keep the card, you will have to present it every time you want keys made. You lose it, you are out of luck, and can never get keys made to that lock again. Unless you bring it in and have it rekeyed, which can be expensive.

Conversely, the authorization sheet allows you to name any and all people that will be allowed to come into the shop with a valid I.D. and have keys made. You, your wife, brother, yourself only, etc......These are the ONLY people that will ever be able to get your key made. Anywhere. Period. VERY secure.

Multi lock offers many different keyways, meaning the milling on the key itself, and this determines whether or not a certain keyway will even go INTO a certain lock.

An online retailer may sell you a multi lock with any number of different possible keyways. Some are more restricted than others. They will probably just issue the CARD to you with the lock. Then, if anyone gets your card somehow, they could go to any MTL dealing lockshop and have keys made. Not as secure. If you go to a locksmith, you will likely receive their restricted keyway. A keyway that has been assigned by multi lock to their shop ONLY.

If you decide to buy a multi lock, and you want to gain the maximum security and key control from your purchase, you should buy it from a local MTL dealing locksmith.