What is the difference between a shunt and undervoltage trip for the breaker?

MN undervoltage release
The MN release opens the circuit breaker when its supply voltage drops to a value
below 35% of its rated voltage Un.
Undervoltage tripping, combined with an emergency-off button, provides fail-safe
tripping. The MN release is continuously supplied, i.e. if supply is interrupted:
either voluntarily, by the emergency-off button,
or accidentally, through loss of power or faulty wiring,
the release provokes opening of the circuit breaker.

MX shunt release
The MX release opens the circuit breaker via an impulse-type (u 20 ms) or
maintained order.
Opening conditions
When the MX release is supplied, it automatically opens the circuit breaker. Opening
is ensured for a voltage U u 0.7 x Un.

Released for:Schneider Electric Australia