The outline of a body is show, transparently. It need not be a cube, but the cube enables these graphics.
Four differential masses are shown: red, green, blue and yellow.
Each differential mass has a position vector from the inertial frame.
Each differential mass has an external force vector acting upon it.
The slider "manifests" the integration process. As one moves the slider, more mass elements are added.
When the slider reaches its maximum, the body is integrated, the position vectors disappear, the force vectors disappear and a total force vector appears, acting on the entire body..
The force vector, in this case, acts on the center of mass of the body