When you create a Turnitin assignment you can select which source the assignment should be checked against to create the originality report. In most cases, all three options should be selected:

  • Student paper repository
  • Current and archived web site content
  • Periodicals, journals, and publications

By default, all student submissions are also added to the standard paper repository. This is the suggested setting.

You can choose not to store the submissions in the student paper repository. This is recommended if you create two or more assignments with similar or identical versions to be submitted at different times. For example: Essay ‘Draft Submissions’ (Assignment #1), and Essay ‘Final Submission’ (Assignment #2). You can choose not to store the submissions for the first assignment, but choose to store the submissions to the standard paper repository for the second assignment.

Read more on Canvas Community:
How do I change Turnitin settings for an assignment?