Apple is now selling SIM-free iPhone 6S and 6S Plus models in the U.S.
The SIM-free iPhone is unlocked, so it doesn't tie you to a carrier or multi-year carrier contracts, giving you more freedom to choose a plan. But the main difference between SIM-free and contract-free unlocked iPhones appears to be the network bands supported.
The SIM-free iPhone 6S carries the model number A1633, and the larger iPhone 6S Plus is A1634.
According to Apple's site, this model is best for AT&T and all the other smaller carriers like Cricket or US Cellular, because it supports their LTE networks.
The other version available globally -- and the model you will get with Sprint, T-Mobile and Verizon -- is the A1688 for the iPhone 6S and A1687 for the iPhone 6S Plus.
This model is listed to support the LTE networks in countries such as Canada, the U.K., France, Germany, Hong Kong, Japan, Singapore, and Australia.
In China, you'll want the third version -- A1700 for the iPhone 6S and A1699 for the iPhone 6S Plus, which supports the country's three big carriers China Mobile, China Telecom and China Unicom.
With the iPhone 5S and iPhone 6, Apple started rolling out SIM-free models several months after initial sales began. This time, it's only been two weeks since the iPhone 6S became available.