Tax exemption option cannot be configured individually for each plan. You can either enable or disable it globally for all your plans. To do this:
You can also choose whether to display customer's VAT number on a user's invoice. If the Display it as VAT Number option is checked, VAT Number will be displayed as a label, instead of Tax Exemption code; If the Always reject, store only option is checked, VAT number/exemption code will be stored, but marked as rejected (though rejection signs in UI are suppressed), taxes will be collected - but VAT Number will be recorded.