GL Sub-accounts By Name
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”.
Note: Do not include the Account Number when mapping by name.
GL Sub-accounts By Number
Accounts in Manage can be mapped to Sub-accounts in QuickBooks Online simply by imputing the Account Number linked to the sub account in QuickBooks Online. QuickBooks Online supports sub-accounts up to 5 levels deep. To track how many levels deep you are you will need to look at your QBO account, because when mapping with Account Numbers in Manage you will want it to look like "1001" and NOT like "1000:1001".
Note: Do Not include Account Names in Manage when mapping by numbers. You will still want to include the name in QuickBooks Online.
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 QuickBooks Online to show the new account. More on this here