Multiple Branch Circuits, are just that, more than one. This usually comes up when multiple circuits enter 1 box. Specifically to 210.7 are terminated to the same device. This question comes up from time to time, and it usually has to do with a garbage disposal and dishwasher on the same duplex outlet, being fed from 2 circuits in a single box. This is a great way to save space, and something not called all the time by AHJ's, but eventually you'll get called on this. "I've been doing this for 20years", "when did they add this code?" Common things we hear. I can't tell you when, but I have NEC books back to 93 as "multiwire branch circuits" under 210-4. Not sure why 210.7 was specifically necessary as it's covered in my understanding in 210-4. But there is some confusions as to the definition of multi-wire and multiple branch circuits. Multiwire assumes 1 conduit or NM cable, as to where multiple branch could be assumed as 2 individual branches, conduit's, NM etc...

Regardless what side of the argument you're on the reason for common disconnecting means remains the same. What's the big deal? Well very simply it's safety. What the NEC is all about. most untrained people, like an appliance installer, wouldn't understand that outlet is or could be supplied by two circuits, on two separate breakers. If this code didn't exist or was adhered to, you would turn off the single breaker you thought was associated with the 1 device you are focusing on, pull the outlet and get zapped by the other circuit feeding it. To complicate it, there is usually poor access to these in the back of a kitchen cabinet and tangled between some plumbing. With a common trip, 2pole breaker, or 2 single breakers with a handle tie, you are forced to shut both off.

Many times in the argument or discussion of these codes, the intent it lost for sake of argument. When I read the code, specific to something new I don't fully understand, I'm always looking for the intent, why is this important? More specifically why is it dangerous? Since the NEC is, in my mind, foremost about safety. A question we should all ask is, what are the safety implications, and more importantly for the electrician, what are the liability implications.