Why these three numbers and not others
Days supply tells you the size of the problem. It answers whether you are carrying too much, too little, or the right amount for the pace your floor is actually selling at. Most stores track it, or a version of it, and most track it badly because they use last month instead of a trailing window.
Auction cost tells you what your default solution charges you. The hammer price is the number you remember, but it is not the number you pay. Buyer fees, transport, the extra recon an unknown unit needs, and the floorplan days while it sits in the shop all stack on top. Most stores feel this cost without ever measuring it.
Max offer tells you what you can do about it on a specific vehicle. It is the only one of the three that governs an individual transaction. Days supply is a plan, auction cost is a channel average, but the max offer is the number that decides whether this car, at this price, makes gross or eats it. It is where money is actually made or lost.