Based on who your mama and papa are..
So if you got a black papa, and a black mama you're URM
If you got a black papa white mama, you're URM(generally)
If you got a white papa and black mama you're URM(generally)
If you got a hispanic mama and papa you're URM
If you got a white papa and hispanic mama(you probably won't be classified as URM, generally biracial latino/white children look more white)
If you got a white mama and latino papa you're probably an URM
If you are Asian, or White, Indian, Arab, etc.. You are not URM in any manner
Oh and Native Americans are URM too
Basically URMs are ethnic groups that are "underrepresented" in medicine. So in order to promote diversity and give opportunity, URMs are given special consideration, even if their application is mediocre at best. (economically disadvantaged perhaps, or other things, etc..)