Hi. These common clamps bite the NM wiring inside a box :


These clamps can bite the wires outside or inside a junction or breaker box, :


----> Is there a mandatory install rule ? The nut can be inside or outside ?

 

If you're going by the NEC ( National-Electrical-Code in the US ),
the answer is NO.
And, it's a bad idea even if your job will never be inspected.

Your job will immediately be failed by any Electrical-Inspector who sees it.
And then he will really start digging to see if he can find
any other "what were You thinking when You did this" "unusual-solutions" on your job.

This is not an approved method of usage for that particular Device.

Don't come up with your own "unusual-solutions", they have a bad tendency to start fires.
.
.
.

 

If you're going by the NEC ( National-Electrical-Code in the US ),
the answer is NO

Your answer is to kind of an either/or question. Just to clarify, I believe the answer is clamp outside the box/nut inside.

Putting the clamp inside seems like an invitation for shorts and wire damage.

 

I have done both and never had any problems or ever failed an inspection.
Certainly there are instances where having the clamp portion inside could cause a problem, and certainly an unskilled inattentive person could have a problem. But the unskilled and unknowing are also prone to making rather deadly errors as well.
It will definitely be a poor choice when using a very small box with a very large component, such as a GFCI.