Why are there different models for different carriers?

[Elektrofone]

So when I am making a reservation it says that there are models available in 128GB Space Grey for Verizon but not for T-Mobile. I thought that the iPhone was unlocked. Are there different models for different providers?

 

[ohio.emt]

The Verizon model supports CDMA, and the now CDMA supports LTE band 30 which AT&T is supposed to be using in the near future for extended coverage.

 

[CodeSpyder]

The AT&T version supports CDMA as well. http://www.apple.com/iphone-6s/specs/

 

[The19th]

The Verizon model supports CDMA, and the now CDMA supports LTE band 30 which AT&T is supposed to be using in the near future for extended coverage.

This is not completely correct. Verizon uses CDMA for voice and LTE for data, and is rolling out Voice over LTE (VoLTE). AT&T is currently the only US Carrier that is rolling out Band 30 for LTE data. The iPhone 6s models for AT&T (A1633/1634) support Band 30 and CDMA Rev. A. The Verizon iPhone 6S models (A1687/A1688) do not support band 30.

 

[Elektrofone]

So how many different models are there? Can I use a Verizon model with T-mobile?

 

[The19th]

So how many different models are there? Can I use a Verizon model with T-mobile?

In the US, it looks like there are three 6s models and three 6s Plus models. A1633/1634 for AT&T, A1687/1688 (GSM) for T-Mo, and A1687/1688 (CDMA) for Verizon. The model information can be found on Apple's iPhone 6s technical specifications page.

Until the hardware gets into people's hands, we're basing it off that information. But yes, it looks like the Verizon models (A168x) can be used on the T-Mobile network, which uses LTE Bands 2, 4 and 12, as do the AT&T A163x models.

There's also quite the discussion in the other post here on MacRumors:
https://forums.macrumors.com/threads/iphone-6s-lte-30-band-and-cdma-version-megathread.1914399/