The Form I-20 is a letter issued by the university to support your student Visa application.
Upon submitting your acceptance and payment deposit to your chosen institution, the institution’s Designated School Official (DSO) will then issue you an official form called Form I-20 to proceed with your student Visa application.
The Form I-20 contains information for you to proceed with your I-901 SEVIS fee payment, including information such as your personal details, school, program of study, financials and so on.
You need the Form I-20 for your student Visa application, whether it is to extend it or apply for a new one.