Looking at the Electronize picture, the unit has four relays, each having a single change-over contact. Each set of contacts has three terminals, marked NO, C and NC. Unpowered, C and NC are connected, when the relay is operated, C and NO are connected. The contact doesn't care whether it is in the positive or negative line to the item being controlled, IF it is providing power to said device. It might be that it is going to control a device by connecting two leads from the device together, without you needing to either know or care what polarity either of those leads are.
In the case of the 2 relay type, it just has bigger relays that can handle more current.
If the contact is going to supply power to the controlled device, and is inserted into the positive supply, the negative needs permanently wiring to battery negative, possibly through a terminal strip. In trade jargon, a bus-bar, "bus" deriving from "omnibus", old meaning, "carrying everything", so it's a bar that carries all the electric. Industrially, it would be a chunky strip of metal with terminals for connecting wire.
