You should use Form G-639, Freedom of Information Act/Privacy Act Request, when you need to gain access to information pertaining to yourself, namely to USCIS immigration records on you, as allowed by the Freedom of Information Act (FOIA) and the Privacy Act (PA). In some cases, FOIA also provides access to Federal agency records. Form G-639 can be used to correct/amend these records. You may also use this request to consent to another person accessing immigration or agency records relating to you.
Besides, with the help of a FOIA request you may find out information concerning your family genealogy, obtain records regarding your family members who immigrated to the USA previously.
Please do not use this form for other purposes, for example, to determine status of any pending applications, to obtain proof of status or a certification of nonexistence of a record. It is considered as incorrect use of Form G-639 online.