OP, the earliest responses are now out of context, off-topic, because you completely changed the opening post. The discussion is now incoherent. Visitors of this thread won't understand what they're talking about. Next time, you can (1) edit the opening post with an additional question while not removing the original post or, (2) if related, you can reply to a response while raising another question, or better yet (3) open another thread for another topic.

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. If you only want to delete your account and you don't have any sub-account, then disregard that option.