The USFID is your unique ID number at the University of South Florida.
Your USFID helps the University provide you with services and helps to better identify you without the need to disclose your SSN to others.
(Read more about USFID: http://www.registrar.usf.edu/regurl/fs/USFID
Your USFID is not the same thing as your NetID. (Read more about NetID: http://it.usf.edu/services/netid