The RCD-24 series is step-down (buck) converter. You always need a higher input voltage than the output voltage required. A high power LED usually has a forward voltage drop of around 3.4V, so only three LEDs can be drivenin series from a 12V source. If the input voltage is increased to 24V, seven LEDs can be in the chain and if the input voltage is 36V, then ten LEDs can be driven.
However, it is possible to connect chains of LEDs in parallel, so a RCD-24-7.0 can drive two chains of three 350mA LEDs connected in parallel.
Remember that this part is a constant current converter, so the output voltage is variable depending on the power needed, but it will always be lower than the input voltage.