Form N-400, Application for Naturalization, is used by lawful permanent residents (green card holders) to apply for U.S. citizenship. It collects information about the applicant’s background, residence, physical presence, and eligibility to naturalize.

ChatGPT said:The N-400 form is used to apply for U.S. citizenship through naturalization.

It’s used to request a certificate of citizenship if you're eligible through parents.