You're referring to the reformed formula approach, I think. In that system, a fuel consumption standard or target is established for each vehicle in the market, ranked by size. The standard becomes, in effect, a formula that is composed of all the possible vehicles in the market, and a company's obligation under that standard is determined by the average of the number of vehicles they sell at each size.
So if they sell 10 large vehicles and 10 small vehicles, it's the average of those fuel consumption levels per vehicle. It's the average across the total number of vehicles they sell in the market, as opposed to one single number at the moment.