The first sum demand account you open after registration. Several powers attach to its bank.
The distinction is administrative until it is not. The bank holding your main account is the one the system treats as your bank: it is the only one that can form a request to suspend debiting across everything you hold, and it is the channel through which you can see the accounts you hold elsewhere. That is a reason to think about which bank gets the first account rather than letting it fall out of whichever opened fastest. (Bank Accounts Instruction, Paras. 36–37 — lex.uz ↗)
Moving the main account to a different bank later is provided for and is not difficult, but it is a procedure with its own steps rather than a change of address. (Bank Accounts Instruction, Para. 39 — lex.uz ↗)
Answer a few questions and the full cost — one-off and monthly — is on the screen in minutes.
Part of the answer bank — 195 questions, each cited to the article it rests on.