Just choose which country you want and click the button. Pick whichever location is closest to you, or actually closest to where the content or service you are reaching sits -- for most people that is the same thing, but it is not always, and the account works the same regardless of which one you choose.
SSR is an older fork of Shadowsocks that stacks protocol and obfuscation plugins on top of the cipher. It is kept here because a few carrier-specific configurations still only work with it.