The items classified as Ancient Documents are not strictly speaking limited to 'letter documents' (which notably are those documents written to have both a sender and an addressee, and with some kind of specific goal or purpose in mind), but also include diaries and other kinds of documents of record.
古文書の部に分類されている物件には厳密な意味での「文書」（特定の発信元と宛て先があり、何らかの目的を達するために作成するもの）だけではなく、日記などの記録類をも含む。