A Whitetail is a airframe already been built for a customer but for some reason didn't get accepted or a something fell through to cause the airframe to not be placed with a customer. So it then sits in the manufacturer possession until they find a new customer for it. While it sits, it's unusually painted all-white.... Hence the term whitetail. United and Alaska picked up a ton of Max whitetails that were rejected by Chinese air carriers after the MCAS groundings for cents on the dollar back in 2022 is the most recently example....

Spirit's Airbus deferrals will not be whitetails as they haven't started the production process yet therefore wouldn't be considered a whitetail.