Allowed Systems and the Center of Mass 
The concept of position can only (by definition) apply to a single point. Because of the central role of position in the One-Dimensional Motion with Constant Velocity model, the template for this model states that it is restricted to describe only a single point particle.
When an object is moving with constant velocity, however, the majority of the object will usually be moving with the same velocity. Consider, for example, a truck moving down the road at a constant speed. You could describe the position of the truck by any given point on the truck's frame: the driver's side door handle, the right front headlight, the rear license plate, etc. No matter which point you pick, you will find essentially the same motion, with a small offset of the absolute position. The velocity of any of these points is identical. Thus, even though a truck is certainly not a true point particle, it can be thought of as one for the purposes of this model. Further, essentially any point on the truck can be chosen as the location of the "point particle truck", but because later models will require us to think of the linear motion of a body as the motion of its center of mass it is advantageous to begin training ourselves now to think of the truck's center of mass as the appropriate point at which to label the truck's "true" position.