These subaccounts is used to detach asset and trading. Asset can be traded among ace and sub-accounts, sub-accounts customer can trade with asset in sub-accounts so to speak. Asset can't be pull over from sub-accounts direct.
Note: The trading fee level of sub-account will be updated at 10:00AM(UTC+8) the next day by aligning with the master account.
Source: https://www.kucoin.com/news/en-kucoin-introduces-sub-account-feature