Hydrocodone is a CIII and Oxycodone is a CII... the CII is more controlled and cannot be filled in most states without a handwritten rx that is sealed.
Because of the level of the drug with the FDA as a controlled substance.
It has to do with the FDA schedule that the drugs are on. Hyrocodone and codeine are Schedule III narcotics which can be called in by a doctor. Percocet/oxycodone is a Schedule II narcotic which means that it's a step up on the ladder from hydrocodone in strength and therefore cannot be called in.
By the same token, if a doctor prescribes, say, 50 Vicodin or hydrocodone (or any Schedule III narcotic) for you and you cannot afford to buy the whole prescription at once, you CAN buy whatever you can afford, leave the rest at the pharmacy, and go back at another time to pick those up. With Schedule II narcotics, if you cannot afford to purchase the whole prescription at one time, you must forfeit however many you cannot afford to pay for at the time.
Schedule III narcotics CAN be refilled, Schedule II narcotics cannot and need a new, handwritten prescription each and every time you get them.
Hope this helps.