The voltmeter shows the voltage status of the battery. The range of the voltmeter is 8 to 16v.
The gauge is supplied a 'U'-shaped fixing bracket and thumb nuts along with a 12v /2w illumination bulb. Night illumination is classic white.
The gauge size of 52mm denotes the outer diameter of the main body of the instrument and hence a 53mm to 54mm diameter hole is required in the dashboard.
Wiring instructions here.