I tried to improve this page providing explanations of the examples, generalize the content, correct some minor errors.
As of october 2018 there is no more 'Categories and items' to click in Grades. Also, the image https://docs.moodle.org/36/en/images_en/6/68/neweditcalculation.png is outdated. Probably https://docs.moodle.org/36/en/Grade_calculations#Setting_a_grade_calculation should be rewritten.
I wanted to add the exactly same comment. Click the 'Categories and items' is not even a proper phrase IMHO.