One of 3 things. 1. Designated prelim years are for people who matched into surgical subspecialties or another specialty that allows you to choose your intern year and you chose to do a surgical intern (anesthesia, radiology). 2. You match into a prelim spot because you went unmatched and you reapply the following year. 3. You match into a prelim spot because you went unmatched and you fall into an opening in the program where you are a prelim (someone leaves, goes into lab, etc...).