Voltage Reducers and Voltage Limiters are two terms with the same meaning. They are used in conjunction with two 8 volt batteries to reduce the voltage from 16 VDC nominal to approximately 13 VDC. This allows two 8 V batteries to power 12 V loads without applying over voltage to those loads.
A Voltage Reducer/Limiter is a low cost solution when minimal use of 12 V loads is expected. For example a vehicle equipped only with lights and expected to be driven mostly during daylight. SPS PN 20434 is rated for 16 ADC Continuous and 20 ADC intermittent.
Heavy use of 12 V loads using a Voltage Reducer/Limiter will over stress the two batteries and lead to premature failure of the battery pack. In the case of radios, frequent use of lights, and other high drain 12 V loads a DC/DC Converter is the appropriate product.