It is not very clear to me when one should choose to add a description or to describe several outputs in the same description.

This is something that has to be tackled conceptually, from the author of the DMP and other research plans. If those outputs have their own lifetime and rules (IPR, Access rights, etc), they should be described as different descriptions. Alternative formats should not be treated as different descriptions, unless they have other differences due to the format, too. But, for instance, if you have descriptons in CSV and JSON formats and under the same terms, they could be seen as one description in the DMP or other research plans.