Similarity Checker vs. Final
Turnitin
The difference between the 'Similarity Checker' and the final submission point
On most module pages, you are provided with a 'Turnitin Similarity Checker', which is a draft space that allows you to check your work before you submit and can be used multiple times without it being uploaded to Turnitin's database. (This is important to remember and is expanded on below)
An example of what you see on most module pages:

When you upload your final submission, it is then sent to Turnitin and stored in their database, this serves two purposes, the first is to make sure that other students can't copy parts of your work, and the second is to make sure that another student can't upload your full assignment themselves without it being flagged.