Is it necessary to select a carrier when buying an iPhone online from Apple? I am planning to buy an iPhone X during an upcoming USA trip. During the buying process on the Apple Store, I am asked to select a carrier between AT&amp;T, T-Mobile, Sprint and Verzion. I dot not have any sim from the USA and I am not planning to have one for using the iPhone X ever in the US. 

Is this important to activate the iPhone?
The reason for this is that in the US carriers have and use slightly different technologies and different frequencies than their competitors. Though I believe T-Mobile and AT&amp;T use similar technology and frequencies. Not sure abut the other two.

I would suggest checking the website of the carrier you use at home and see what they use so you can select a phone that is the most compatible with their cellular network.