Anyway, the option is more or less self-explanatory. If you have multiple accounts or sub-accounts, it's for you to combine them. Merge is to combine. Multiple accounts means, for some reasons, you are maintaining more than one account, sub-accounts to be specific because multiple accounts aren't allowed in Binance.
Has anyone tested this in practice?
1) Which account will disappear as a result of the merger? There is not specifically stated
2) What will happen to the balances? The merger will mean the disappearance of one of the accounts.
3) Will the KYC on main binance acc fly off during this merging procedure?