The Victron Shunt 2000A - 50mV (MPN SHU202050200) is needed for current measurement devices with analogue or digital displays as these are normally realized by a measuremnt of a very small voltage on the shunt. In the case of Victron Shunts 1000 - 6000A this is a voltage of 50mV. Generally spoken: The smaller the measuring voltage on the shunt is the smaller are influences on the rest of the system's circuit. 50mV provide an excellent outcome.
For this purpose the shunt is integrated into the conduction where current has to be measured. The small voltage dropping at this resistor is evaluated by a measuring device - like the Victron BMV 700 battery monitor - which then displays the current amperage.