Wire Pair versus Signal Path
A wire pair (positive wire and negative wire) is used in the electrical circuit providing power to the horns and strobes. On the other hand, the NFPA's definition of a signal path includes wire, fiber optics, and radio waves (Wireless or RF); the signals travel along a wire pair path, a fiber optic path, or even with an RF path.
A signal, here, is "Turning on the horns and strobes", so here, the pathway is the pair of wires that carry power to the horns and strobes.
Class A Circuit versus Feed & Return
One way of thinking about a Class A circuit or pathway is to consider that a "Circuit" is a "Circle". The Class A pathway (pair of wires) starts out at the panel on the Class B output, goes to the first horn or strobe, to the next, and the next, and finally back to the panel. It then connects to the Class A input. This is a complete circle.
If the circle (Class A pathway) is interrupted, like a wire comes loose or breaks, all the devices are still connected to the panel on one end of the circle or the other.
The reason for the two separated routes for the Class A pathway (feed and return separated) is to make sure that whatever interrupted the pathway, doesn't interrupt both the feed and return parts of the pathway. A forklift hitting a junction box, for instance, could break all the wires in the junction box.
If both the feed and return pathways go through the junction box, both the feed and return pathways would be interrupted, and any devices beyond the broken junction box are now disconnected from the panel.
As long as the feed and return are separated, the forklift could only break the feed wires, or the return wires, but not both.
That's a single Class A pathway.
A Pathway is Not a Conduit
The pathway does not require a conduit. From another point of view, a conduit is just some metal wrapped around a circuit or pathway. The conduit could be wrapped around wires, it could be wrapped around fiber optics, or it could be wrapped around both. In other words, the conduit could be wrapped around several pathways.
If each separate Class A pathway is installed through the building so its feed and return wire circuits take different routes, multiple Class A feeds can be wrapped in one conduit, and multiple Class A returns can be wrapped in the other conduit.