Apple iPhone 6s/Plus country availability map and LTE bands compatibility
Apple iPhone 6s and 6s Plus rollout schedule and country availability map
The new iPhones support up to 23 different LTE bands, depending on the model, including all the TD-LTE ones found in places like China or Japan. That's a record for the phone industry, beating the previous iPhone 6 record by three extra bands. This way you can use your US-bought iPhone 6s and 6s Plus all over Europe and Asia with fast download speeds.
Of course, Apple is cautioning that it depends on individual carrier settings, saying that "unlocked models may support LTE networks outside the country of purchase when using a valid SIM from a supported carrier. Contact your carrier for more details," but if history is any indication, most of the times you can just pop a local LTE SIM card if you've bought your device unlocked, and enjoy the blazing download speeds that the local carrier in question offers you. Here are the countries and carriers each different iPhone 6s or 6s Plus model number supports.
FCC OKs Cingular\'s purchase of AT&T Wireless
Things that are NOT allowed: