Step up DC-DC transformers are used to increase the voltage to a load from a lower voltage source. For example, running 24V loads from a 12V battery.
Step down DC-DC transformers are used to decrease the voltage to a load from a higher voltage source. For example, running 12V loads from a 24V battery.
This line of DC-DC transformer is best used when the source voltage is close, or could vary above or below the regulated output voltage. For example, the FB 12-3 could be used to power a load at 12V from a 12V battery. The 12V battery voltage can range from 11-14 volts and the unit will always regulate at 12V on the output.