Your boss should provide the following information in particular:
The start and end date of the employment relationship
Your role and the activities you carried out
Your work-time percentage
An assessment of the quality of your work
A conduct assessment
The reason for issuing the reference letter (termination, change of direct supervisor, change of role, etc.)
Your employer must give you the employment reference letter in writing.
It must provide a full and honest description and a non-acrimonious assessment of the employment relationship. If you find inaccuracies, errors or falsehoods in the reference letter, you have the right to ask your boss to change the text.
The document should not contain any unclear or coded language that could be interpreted in various ways by the reader.
Nor should there be any mention of isolated negative incidents (e.g. one time where you were late for work) or of facts not directly related to the employment relationship
The State Secretariat for Economic Affairs (SECO) provides a list of FAQs (web page available in German, French and Italian).