The background in two sentences
Under the German Federal Vacation Act, vacation is to be taken in the calendar year, and carried-over vacation usually by 31 March of the following year (§ 7(3) BUrlG). Since the European Court of Justice's Max-Planck ruling (C-684/16) and the Federal Labour Court's decision of 19 February 2019 (9 AZR 541/15), vacation generally only expires if the employer has first told employees about their remaining days and that they may lapse.
What this means for your company in detail is a question for your legal adviser. This page describes how Chrontic handles the notice, the carry-over and the record.