Hi Chakhadrar,
The tax indicator is not assigned to a sales area but to a country. See the key of table KNVI which keeps this information.
When you maintain the sales data in customer master in a certain sales area then SAP looks up all assigned plants and their countries to this sales area. It then shows all tax types assigned to the plant. That is the reason why a change in one sales area is affecting other sales areas.
If in your case the tax determination depends on the sales area then you access sequence assigned to the tax condition types (i.e. MWST) should deal with this by finding different tax keys depending on the sales org. You could i.e. use a condition table with the key country, tax indicator, and sales org and setup records like
UK 1 T1
DE 1 T2
(it's likely that the whole setup is a little bit more complicated).
Best regards,
Oliver