frontier worker
as defined in Regulation (EEC) No 1408/71 of the Council of 14 June 1971 on the application…, Article 1, point (b)
any worker employed in the territory of a Member State and residing in the territory of another Member State to which he returns as a rule daily or at least once a week; however, a frontier worker who is posted elsewhere in the territory of the same or another Member State by the undertaking to which he is normally attached and is prevented on account of such posting from returning daily or at least once a week to the place where he resides shall nevertheless retain the status of frontier worker for a period not exceeding four months