Writing my first Letter of Recommendation for a student. What's with the "Strength of Recommendation" drop-down list?

I'm a graduate student teaching associate who's been asked to write a letter of recommendation for my student. When I go to their portal to upload the letter, it has an empty field just above the file drop box. The field says says "Strength of Recommendation," and clicking on it lets me select an option from a drop-down: "Do Not Recommend," "Recommend with Reservations," "Recommend," "Strongly Recommend," or "Highest Recommendation Possible."

This is my first time submitting an LOR for a student, so I'm not sure what this dropdown is for. Is this something I fill out as the recommender, or something the application reviewer fills in after reading my letter to summarize its evidence/quality? This student was truly fantastic and memorable, so I'm worried about messing this up for him! I can leave it blank, but not sure what's best.