The summary has to contain all the main ideas in the summarized text.
Just like a diagram, it has to be based on the outlining, and it has to be personal.
Ideas have to be in a hierarchy, and they have to be related to each other.
As a guideline, the summary shouldn’t be longer than a fourth of the summarized text, although sometimes it depends on the complexity and conciseness of the text, its technical character, etc.
It must go from general to particulars, and it shouldn’t include many details.
It’s useful for students who have a good audio memory.
The summary should be formed with very few points, and they don’t have to be close together, because they make it too long.