What does "For disconnect use only" mean on a connector?

The French means 'do not use to break the current'.

In other words, this connector is not to be used as a switch. Only pull them apart when the circuit is drawing no current.

With a 50A rating, and used with DC, breaking the connector circuit is likely to cause an arc as the contacts separate for long enough to damage them.

"For disconnect use only"

should read

"For disconnected use only"

in the sense of

"Only 'use' this socket (i.e., 'put a plug in or out of this socket') when the the power is off."

A good translation would perhaps be

"Do not use this plug as a switch"

or

"Do not use unplugging as a means to turn off."

or what about

"You must turn off the power, before plugging or unplugging this plug."

The basic confusion here:

If you say you are "using" a socket, it usually means that, "today, all day, something is plugged in to the socket". The socket is "in use", it is "full", it is "being used by appliance X".

But the other way to "use" a socket is literally the act of plugging in and plugging out. (As in "use a screwdriver" or "here, use this".)

in this sentence:

"Only 'use' this socket (i.e., 'put a plug in or out of this socket') when the the power is off."

The 'use' means 'the action of putting a plug in or out of this socket'. That's what's going on here! It's somewhat clearer what is meant in French.

I have worked as an electrician for over 20 years in the U.S. I also hold an associates degree in electrical technology. This is a very common terminology. It means that this only acts as a "disconnect", which means you use this to "disconnect" the power, to either work on the equipment, or as a way to wire in portable equipment. As opposed to a breaker, which means that it is both a disconnect and an over current device, which automatically stops all current from flowing when it pulls too many amps. This is there to stop the wires from overheating in the case of a malfunction. Also from my high school french class I learned that when "ne pas" is used it means "not". As in the statement being expressed is not true