Hi, the letter we were discussing in class is described here (page F40 of the manual):
Certification Letter for Predoctoral Fellowships (F31) to Promote Diversity Applications submitted for individual predoctoral fellowships (F31) to promote diversity in health-related research are required to attach a certification letter (titled Diversity_Eligibility_Ltr) from the institution certifying eligibility of the fellowship applicant for the program. The letter should avoid revealing sensitive personal information, such as the candidate’s specific racial/ethnic background or type of disability. The certification letter must be on institutional letterhead and scanned so that an institutional official signature is visible.
Basically the letter would say you are an under-represented minority.