If you would like to break down the marks you give to students into discrete sections you can use Turnitin Grading Forms.
To access these you click on the fourth icon to get to the Rubrics section. Then click the spanner icon in the top, right of the rubric grading panel to the right of the document (just below the grade box) and add a new Rubric or Grading Form.
Then click in the top, left corner of the pop-up box and choose "Create new grading form" 
Fill in your section titles and (optional) descriptions, then click the link icon in the top, right corner of the pop-up box to attach it to the assignment. You only need to do this for the first paper you mark and it will be there for the rest. 
