Using the definition of equivalent systems, you can replace one set of loads with another set. If your problem is two dimensional, I can replace all the loads in your problem with a single force as long as you allow me to pick the magnitude of the force, the direction of the force, and the point of application of that force. In three dimensions, I can't necessarily do that. But I can replace all the loads in your problem with a single force and a moment that lies along the same line of action as the force. What do we call this reduction of a system of loads