If you're applying for a surgical residency—General Surgery, Urology, Otolaryngology-Head & Neck Surgery, Neurosurgery, integrated Vascular Surgery, integrated Cardiothoracic surgery, or Plastic Surgery—it's almost a given that you'll be doing a sub-internship, or a "sub-I." (and sometimes called an "AI" or an acting internship). That being said, many other specialties such as Radiation Oncology or Dermatology, encourage doing away rotations/sub-Is. Sub-Is are obtained through VSAS, or the Visiting Student Application Service, and are usually for a month. You apply for institutions participating in VSAS towards the end of your third year.