Once you deactivate a Tax region below things will happen:

1. You no longer will be able to associate clients with an Inactive Tax Region. That is any new client you create should belong to an Active Tax region
2. You no longer will be able to associate a document with an Inactive Tax Region. That is any new document you create should belong to an Active Tax region
3. All the previous clients & documents created with an inactive Tax region will still remain in the same region and continue to use the corresponding tax rates while calculating taxes for the line items.

Once you deactivate a Tax category below things will happen:

1. You no longer will be able to associate any products with an Inactive Tax Category. That is any new product you create should belong to an Active Tax region
2. All the previous products created with an inactive Tax region will still have the same catgeory and continue to use the corresponding tax rates while calculating taxes for the line items.