By measuring the voltage drop across a shunt, the VA1-XX can determine how much amperage is being loaded on the aircraft's battery or alternator/generator.
The VA1-XX requires an external shunt. For the VA-1A-XX, the "XX" equals the value of the shunt. Most people who are replacing their existing Voltammeter will replace it with a similar type (internal or external). Internally shunted systems generally have large lugs at the back of the instrument (this is part of the internal shunt), while externally shunted instruments have small wires that leave the back of the instrument.