Currently it works this way:
ApprovalMax does not allocate the next Purchase Order/Bill number as QuickBooks Online does this for us to ensure that the numbers don't get duplicated.
Please note: this is only the case if the "Custom No" option has been switched off in QuickBooks Online.
If this option has been activated, then QuickBooks Online does not assign any numbers to ApprovalMax Purchase Orders/Bills and the respective fields are left blank.