Uni-assist checks whether your high school diploma/school-leaving qualifications grant you access to higher education in Germany and whether you fulfil the Freie Universität Berlin's admission requirements (→step 1). Uni-assist also converts your grades into the German grading system. If you fulfil all of the requirements and have handed in all of the necessary documents, your application will be forwarded to the university and included in the selection process. The university decides on your admission, not uni-assist.
Assessment Examination
You cannot apply directly for Freie Universität's Studienkolleg. You have to apply for a specific study program through uni-assist first. While processing your application, uni-assist will check the higher education entrance qualification (HZB) that you submitted. If your HZB does not grant you direct access to higher education institutions in Germany, uni-assist will evaluate whether you fulfilll the requirements to attend the Studienkolleg. If so, Freie Universität Berlin will invite you to the Studienkolleg's assessment exam. The Studienkolleg's program always starts only in the winter semester.
Further information
... can be found on the university's Application website.