What is the difference between unlocked iPhone 6 and T-Mobile no contract iPhone 6?

So I tried to go on apple website and look for an unlocked version iPhone 6 Plus, but they only have T-Mobile version is without any contract, but it comes with a T-Mobile SIM card, does it means that I have to use this card and T-Mobile service? Can I use other carrier's card instead if I buy this version of iPhone? I travel a lot, like going back to China, I use China Mobile, can I use their SIM card on this T-Mobile iPhone?

The t-mobile verision is unlocked, but it comes with a tmobile sim card, you can put any sim in it thats compatible of course.