Judgment of the Court (Sixth Chamber) of 4 September 2025.#KN v Raad van bestuur van de Sociale verzekeringsbank.#Request for a preliminary ruling from the Hoge Raad der Nederlanden.#Reference for a preliminary ruling – Social security – Migrant workers – Legislation applicable – Regulation (EC) No 883/2004 – Article 13(1) – Regulation (EC) No 987/2009 – Article 14(8) and (10) – Worker normally employed in several Member States – Pursuit of less than 25% of the activity in the Member State of residence – Concept of ‘substantial part of the activity’ – Connecting factors relating to working time and/or remuneration – Consideration of other circumstances – Length of the assessment period – Discretion of the competent institutions.#Case C-203/24.