I think they probably mean the source to fund the trip like ticket? Since you are funded, it is more likely that the assistantship will be explicitly written on your I-20 and you probably won't need a financial certificate/statement. I think if you are paying the flight ticket, you can put "self". Otherwise, if it is paid by other party, you can write down that entity. I am not an expert but this is what I probably understand from the situation as an international student too. And you certainly want to ask your school advisor who can know these very well.