The article describes sub-levels in QuickBooks and its limitation.
Note: This method works for all lists that have sub-levels, except for the Customer:Job list/Customer Center and Vendor List/Vendor Center.
Edit a list entry to make it a sub-entry
Use the List/Center to make a list entry a sub-entry
QuickBooks can create up to five sublevels of items, accounts, and customer:jobs.
If you try to exceed this limitation, you will receive one of the following warning messages: