A trade server name is the third part of a platform login, alongside the account number and the password: it identifies which of a broker's servers that account actually lives on. The number on its own is not enough, because the same number can exist on more than one server and mean a different account there, or no account at all.
Brokers usually run several servers — demo accounts on one, live accounts on another, and sometimes separate servers for different entities or regions — so an account opened under one entity cannot be reached from a terminal pointed at another. The name is also not permanent. Servers are consolidated, renamed and moved, and a terminal that has cached an old address goes on failing to connect while the credentials themselves are perfectly valid. This is why a failed login is not evidence that a password is wrong, and why the first thing a support desk asks for is the server rather than the number.
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