Case C-661/18: Judgment of the Court (Seventh Chamber) of 30 April 2020 (request for a preliminary ruling from the Tribunal Arbitral Tributário (Centro de Arbitragem Administrativa — CAAD) — Portugal) — CTT — Correios de Portugal v Autoridade Tributária e Aduaneira (Reference for a preliminary ruling — Value added tax (VAT) — Directive 2006/112/EC — Deduction of input tax — Article 173 — Mixed taxable person — Deduction methods — Pro rata method — Deduction on the basis of actual use — Article 184 to Article 186 — Adjustment of deductions — Change in the factors used to determine the amount to be deducted — Output transaction incorrectly regarded as VAT-exempt — National measure prohibiting a change in the deduction method for years that have already elapsed — Limitation period — Principles of fiscal neutrality, legal certainty, effectiveness, and proportionality)