Before applying for the above-described authorisations, the applicant also has the possibility to apply for a pre-engagement via the OSS. The pre-engagement is not mandatory for the applicant, but it is mandatory for the safety authority chosen by the applicant. The purpose of such pre-engagement is mainly to clarify how the vehicle authorisation process will work and how decisions will be taken, to explain the rules in force, to define the scope of the application, etc. (For details on pre-engagement, see Chapter 3 of Regulation (EU) 2018/545.)