Data Centre

I don't know for sure, but it seems to pick the the one for you that would give you the fastest connection speed at the time. For example, the closest to me is the East US, but sometimes it gives me North Central US, because for some reason that one is faster than East US at certain times. You can tell by clicking on "Data Center" in the lower left of the main screen where you can see the entire list of servers and see the ping for each.

No matter what server it gives you, you can change it to whatever you want. For example, I normally use East US or North Central US, but if I want to play at 3AM EST or after, I will usually switch to one of your Belgium servers, because there are many more players on there than on the US servers at that time of day, so it is much easier to get into games. Sometimes it is better to pick a slower server if it has more players and gives you a better chance of getting a game.

I've done this in TF1 for a long time now. I would always use East US until about 1AM EST, but then I would switch to North Europe, because even with the user base being so low, I could always get games whenever I wanted by picking the right server at the right time of day.

 

you can select the server you want in the main menu