Secondary regulators are used when you are attempting to dispense two or more kegs off of the same Co2 tank. A secondary regulator will allow you to regulate the pressure of each keg individually. Features 2 pressures.
You still need a primary regulator that then connects to the secondary with a piece of air line. Each secondary regulator has its own shut off valve and adjustment pressure adjustment knob. Secondary regulators are best mounted to a wall or the inside of the refrigerator.