This post courtesy of Jill Fresen:
When using the Assignments tool in WebLearn, the default behaviour is that student papers do get added to the Turnitin database; however, they are used only for comparison purposes and are not released or shown to any other users, apart from the student and instructor concerned. (It may occur in the future that another instructor at another institution may contact you if any of their student submissions match some text in your student’s paper, and then they can request permission to view your student’s paper.)
If you do not want your student papers to be added to the repository, this can only be done when creating an assignment directly in Turnitin via the http://submit.ac.uk interface. There is an explanatory video explaining all about “Creating an assignment” at http://submit.ac.uk/static_jisc/ac_uk_training.html .