From Apple's site:
The SIM-free iPhone comes and not using a wireless agreement dedication or service financing. It doesn’t include a nano-SIM card for iPhone 6s, iPhone 6s Plus, iPhone 6, iPhone 6 Plus, or iPhone 5s. And as it’s unlocked, it isn’t tied to at least one service, so you'll be able to get a nano-SIM from any supported service around the world, equivalent to AT&T, T-Mobile, Verizon, or Sprint in america.SIM-free iPhone 6s vendors the style quantity A1633, at the same time as the iPhone 6s Plus is classified as A1534.
Apple has been selling unlocked iPhones proper from September 25 — the day while the iPhone 6s and iPhone 6s Plus went on sale in the USA — however that is the primary time that it has began selling SIM-free variations of iPhone 6s and iPhone 6s Plus.