The document is explained in detail on the education and training page of the official website of the European Union. I translated it for you:

Diploma supplement

Diploma Supplement (DS), accompanied by higher education diploma, provides a standardized description of the nature, level, background, content and status of the learning completed by its holders.

As an annex to the higher education diploma, the diploma supplementary document (DS) gives a unified description of the diploma, covering the nature of the diploma, education level, study field, study content and the status of the diploma holder's completion of study.

. . . This supplementary material is intended to help gain recognition-it is not a substitute for your resume or original qualifications, nor can it guarantee recognition.

. . . This supplementary document is used to assist diploma certification-it is not a resume, nor can it replace the original diploma, nor can it guarantee certification'

This is the link to this webpage:

http://EC . Europa . eu/education/lifetime-learning-policy/doc 1239 _ en . htm

This is a detailed description of this file format, you can have a look:

http://EC . Europa . eu/education/lifetime-learning-policy/doc/ds/ds _ en . pdf