It is a number, assigned to a hypothetical perfect bullet, under ideal and constant atmospheric conditions, at a constant velocity. There are three major considerations that we MUST look at here. "Perfect", "Ideal", "Constant". None of these apply to any man or environment know to man, or at least inhabited by man.
First, what is the "perfect" bullet? It is a bullet which is three-calibers long, and ogival head of two-calibers radius, and of homogenous construction with equal and concentricity of the mass around the center from tip to butt. Got that? Name one bullet that meets those specifications!
This bullet must be fired from a source that will establish and guarantee that a constant velocity of that bullet will remain from the moment of launch until the moment of impact. Got that? Name one projectile that meets that requirement!
In addition, all this MUST take place at exactly sea level, at a temperature or 59-degrees F., 29.58-inches of mercury barometric pressure and 78% humidity. Oh, and absolutely no movement of the air... Name one place on earth that has those qualifications, 24-hours-per-day, 7-days-per-week, 52-weeks-per-year... Can't, can ya!
Now if we could find that "perfect" bullet, and launch it and maintain it at the "constant" velocity, under the "ideal" conditions, we would be able to assign a ballistic coefficient of 1.000 to that bullet. That's a hell of a lot of work to get a rating of "1"!