How to exclude an AR transaction type from tax?

Hi Experts,
I read on this forum that it is not possible to exclude an AR transaction type from etax if you have configurated a tax regime using global configurator.

How can I undo this? Or, if I want to create a new tax regime/tax which settings should I use to make my tax operating unit specific?

I tried to create a new tax below an existing tax regime but the system only proposes 'global configuration options' and legal entity, so not operating unit.

Can anyone help me out?

Thanks a lot!
Cathy