The "File Number," also called the "A number" is a person's immigration file number. Unlike Receipt Numbers which a re specific to a petition, this number is specific to the person. The format of the number is the letter "A" followed by eight or nine digits (if nine, the first digit is often "0")
Many people simply won't have one until they file some form of petition towards permanent residence. However, they are assigned in other contexts. Someone who was only here as a student, but received Optional Practical training after graduation, will have an "A number" right on the face of the employment authorization card. But, a person who first entered as a visitor, or an on H-1B, who is now filing for adjustment may well have never received an A number, and "none" can be entered for this question.
Consult a lawyer for further guidance on your case.
If the applicant has an A# (Alien number) that can be included on the form. If the applicant does not have an A# you can state "NONE". You can consult an immigration attorney to help you with your petition.