Wikipedia
A "Sub-internship" (abbreviated sub-I), AKA "Acting Internship (AI)", is a clinical rotation of a fourth-year medical student in the United States medical education system which may take place at a different hospital than the student's medical school affiliations. A student will generally select an elective "sub"-internship, where the student will perform the role of an intern or first year medical graduate, under the supervision of senior house staff and attending physicians. The rotation is generally pursued in the field appropriate to career interest. For example, a sub-I can be taken in Internal Medicine, Surgery, Pediatrics, or Obstetrics and Gynecology. In this way, the student can experience a glimpse of their future career without the burden of full responsibility. Additionally, many medical students select sub-internships at institutions where they wish to apply for residency; such rotations are "auditions" for the student to demonstrate his or her commitment to the outside program.