Both Voltage Sensitive Relay and Split Charge Relay are used to manage batteries in a dual battery system, which usually consist of a main battery and an auxiliary battery.
Typically, Split Charge Relay is controlled by the ignition wire, so that when the engine is running (ignition is ON), the split charge relay will close and connect both main battery and auxiliary battery. When the engine is not running (ignition is OFF), the split charge relay will open and disconnect the batteries. While it is relatively easy to install and operate, the charging and discharging operation is hardwired so the system will operate regardless of the state and condition of the batteries.
On the other hand, Voltage Sensitive Relay senses the voltage and thus the charge level of main battery and decide based on the charge level of main battery, whether it will close and connect both batteries or not. This is a superior way of charging, make sure that the main battery is full before it will charge the less important battery (auxiliary battery). It also allow part of the main battery to be used for accessories and less important use, while maintaining the necessary charge in the main battery to crank the engine.
There are also some smart, integrated modules, which include not only battery management but some of the smart switching functions, which allow people to control their accessories easily with touch of a button.