According to Calvin Henry-Cotnam :

[ multiple circuits feeding a single outlet box.]

In residential installations where this has had to occur, I have never
had a problem with inspectors, and none have ever required labelling, but
I prefer to put a label (usually on the inside of the cover plate), just
in case any future work is not done by myself.

Odd.

I thought the CEC and Knight are pretty explicit about _requiring_ that
every box must be completely deenergizable by one disconnect (other than the
main of course ;-) There were some ways you could get around this, eg:

- metal barriers between the sections
- locking the boxes or area so they're not accessible by
"unauthorized persons".

This is why, for example, the main and branch breaker sections of
residential panels are internally segregated by metal, and usually
have separate covers.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.