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. Not specified at Binance?
2) What will happen to the balances? The merger will mean the disappearance of one of the accounts.