Unlike in the Anglo-American world references and recommendation letters are not very common in Germany. But they are a legitimate opportunity to draw attention to personal qualities that are not completely covered by the more formal documents. References and recommendation letters can be written by professors, and supervisors of former employments or internships.
A letter of recommendation must not comply with any formal criteria. The writer guarantees with his/her name for the applicant. Therefore, a letterhead with name, contact address and personal position is required. The letter itself should indicate when and how the author has met the candidate and how they relate to each another. It contains information about the personal qualities, the working methods and behavior in individual and group work. The letter of recommendation is as individual as possible and written in the first person. It should end with an overall assessment of the candidate. It is important that the author of the letter evaluates exclusively positive, even in case of being consulted by the HR officer. No reference is better than a doubtful one. (The latter paragraph is a summery of https://www.staufenbiel.de/ratgeber-service/bewerbung/referenzen-und-empfehlungsschreiben.html)