What is the difference between an unlocked T-mobile iphone7 and a sim free iphone7?

i currently use T-Mobile and my TM sim card is not tied to any device. so if I order an unlocked T-Mobile iphone7 tomorrow, will it be compatible with my current sim card?

Also I travel to foreign countries a lot and I will have to use a SIM card from foreign carrier when I am outside US. So will the unlocked T-Mobile iphone7 be compatible with my foreign SIM card(it is GSM, the same network as T-Mobile)? Or should I instead buy a sim free iphone7 given my case? What's the difference about a sim free iphone7?