Accounts in Manage can be mapped to Sub-accounts in QuickBooks Online simply by delimiting the account in Manage with a colon “:”. QuickBooks Online supports sub-accounts up to 5 levels deep. An account 5 levels deep in Manage would then be in the format “level1:level2:level3:level4:level5”. Do not include the Account Number.
It is important to note that any time that you make changes to your GL Account Mapping you need to open and then close any closed invoices that use that mapping for it to change. Once you have done that you can then successfully run the batch in QBO to show the new account. More on this here.
Since the Account Number field is optional, QuickBooks Online uses the Account Name as the primary key. As a result, the Account Number should never be included in any mappings within Manage.