It's in regards to a written application. "Referred by" means that someone, your Doctor for example, said it was good for you to go to a place or do a thing, "the Dr referred me to the X Ray Department because I think I have broken my wrist". So when I was in the X Ray Department, I had been referred there by the doctor. If I was filling out a form (with a broken wrist? hahaha) I might see "must fill out (this section of questions) if referred by (the doctor - a lawyer etc). So must fill out = you have to write information here you have no choice. If referred by = if you came here because a specific person or organisation told you to come. More or less :)

If someone has recommended or suggested that you write whatever you are writing the form for, you must answer the questions in that particular area of the form. For example, if you are applying for work at a company and you've heard of this company from someone who has referred you to apply there, then you "must fill out" - complete that section of the form. It is difficult to know what it means because this is not a sentence and there is no context for what it is regarding.

im not sure if this helps u understand it, but it basically means u need to fill the form or the space required if someone or something refer you to that? i can give you a better answer if you provide more details