noun
report; declaration; written statement
Refers to a formal written document submitted to an authority, such as a tax return, customs declaration, or official notification.
税務署に確定申告書を提出した。
I submitted my final tax return to the tax office.
You need to fill out a customs declaration form when entering the country.
届出 is a general notification or report, often less formal than 申告; 申告書 is specifically a declaration form, often for tax or customs.
Compound of 申告 (declaration, report) + 書 (document, writing).