A real rotor does not only have a single unbalance, theoretically it has an infinite number arbitrarily distributed along the axis of rotation. These can be replaced with two resulting unbalances (shown here as arrows) in two random planes, which generally have different sizes and angular positions. As this unbalance condition can only be fully determined during rotation, it is called dynamic unbalance. It can be broken down into a static unbalance and a couple unbalance, whereby one or the other can be the overriding unbalance.
Two correction planes are required to completely correct dynamic unbalance. Dynamic balance occurs in virtually all rotors. Therefore, both horizontal and vertical balancing machines are used.