NFPA 70E requires each lockout/tagout device to “be unique and readily identifiable as a lockout/tagout device” [120.2(F)].
How can a lockout/tagout device be “unique”? In this context, the intention is that the device serves only one function. It’s a lockout/tagout device. It is unique in function (its only function is to be a lockout device), even though there may be dozens of such locks on site. We know that because the second requirement is that it be readily identifiable as serving this function. We also know that because the requirement is repeated in 120.3(B) along with the text, “shall be used for no other purpose.”
Another implication here is each lockout/tagout device is unique to the person who installed it. That is also implied 120.3(C)(2) with the requirement that the device include a method of identifying the person who installed it. That person is the only person with the authority to subsequently uninstall the lock, with the exception that under extraordinary circumstances there is a procedure for the shift supervisor or a similar authority to uninstall the lock.
If the padlock on your toolbox is functionally no different from the lockout lock sitting next to it, how do you have a lockout lock that’s readily identifiable as a lockout device? A quick look at what’s available through your local electrical distributor or other supplier may answer that question. A common scheme is for such locks to have a red case. They can also be specified to come with a label that readily identifies them as lockout locks, and such labels are available separately.
At a large facility or on a big construction site, locks may be color coded by trade or department. This makes it much easier to find the owner (the one who installed the lock). So perhaps red for electricians, blue for mechanics, gold for operators, gray for instrument techs, and green for everyone else. Or all of the locks are the same color but each department or trade has a colored dot or stripe on the lock. Perhaps a plain chrome lock is absent from this scheme, with colored locks being designated as lockout locks.
What happens if an electrician runs out of lockout locks? Say each electrician has six locks, and Mike is on a job that requires seven such locks. A common solution is Mike borrows a lock from Jeff, but Mike still hangs his own tag on it. The tag is what tells everyone else which individual installed that lock. What Mike cannot do is use his spare toolbox lock, because that lock is not uniquely a lockout lock.
There is a fair amount of leeway in how a given facility or organization complies with 120.2(F). In a facility with a small maintenance department, having locks color coded to each trade isn’t going to be particularly helpful since anyone reading the tag will know the person who hung the tag anyhow. But the lock still has to be used only as a lockout lock, and people still must be able to easily recognize it as such. NFPA 70E does not say that the tag can serve the purpose of making the lock readily identifiable as a lockout lock; in fact, it says that means must be in addition to identifying who installed the lock [120.3(C)(2) and (3)]. So, make sure something about the lock communicates that message.