With previous iPhones, most retailers just stocked iPhones that were specifically tied to the carrier they were being purchased for. This meant having a ton of inventory. At Best Buy, iPhone 6 and SE still come for specific carriers, with the ICCID barcode on the box and SIM installed in the phone (except AT&T, which for some reason never has the SIM physically installed in the phone). iPhone 6s/6s Plus, 7/7 Plus, and 8/8+ are all ‘universal’ and thus do not come with an included SIM and are only locked to a carrier if an installment billing agreement is drawn up. Same can be said of recent cellular and WiFi models of iPad. I’m just not certain which model iPad Best Buy started selling ‘universal’ SKUs.
It has been a major improvement for Best Buy, as it means less inventory complexities and customer disappoints, especially when the devices are new and harder to come by. You used to run into problems where a customer wanted say, Sprint, but the only phone in the color and size they wanted was on AT&T. In that situation, you either had to order the phone, or the customer left and went elsewhere. With the ‘universal’ SKUs, you grab the color and size you want, grab a SIM for their carrier, and go.
With the ‘universal’ SKUs, a customer can either buy the phone on installments, which locks the phone to that carrier until paid off, or they purchase the phone outright and it’s not locked to a carrier. Best Buy only sells the Qualcomm iPhone 6s/7/8 series, which means there’s no compatibility issues if you want to switch carriers down the road.