Hi,
If you want to keep TAN also then why you created ZTAN?
Moreover you can also maintain ZTAN in MLTCA and while sales order process you can manually changes TAN to ZTAN when required.
The use of usage and higher level item category is use in:
Usage: its a ABAP hard coded program use in specific business application example batch management or contract
Higher level item category is use in process line free good etc.
Anyhow I believe your issue is resolved.
Thanks.
Sumeet Sah