Case C-879/19: Judgment of the Court (Eighth Chamber) of 20 May 2021 (request for a preliminary ruling from the Sąd Najwyższy — Poland) — FORMAT Urządzenia i Montaże Przemysłowe v Zakład Ubezpieczeń Społecznych I Oddział w Warszawie (Reference for a preliminary ruling — Social security — Determination of the legislation applicable — Regulation (EEC) No 1408/71 — Article 13(2)(a) — Article 14(2) — Person normally employed in the territory of two or more Member States — Single employment contract — Employer established in the worker’s Member State of residence — Paid employment activity pursued exclusively in other Member States — Work performed in different Member States during successive periods — Conditions)