<p>“Is it something that only applies to returning students?” Basically, yes. That’s why it says “(if applicable) …” If you don’t have one, you can’t enter it – and you don’t need to worry about it. There may be cases in which an applicant has acquired a Dartmouth ID ahead of time (I have no dea what they might be, but I can conceive of some), but, if you have never received anything from the college with a “D######” [there may be more digits at some point], don’t worry about it.</p>