noun
General term for a document that explains how to use or operate something, such as a user manual or instruction booklet.
新しいソフトウェアの仕様書を読んでください。
Please read the manual for the new software.
noun
specification document; specifications; blueprint
Technical document detailing requirements, design, or technical standards for a product or system. Common in engineering and IT.
開発チームは仕様書に基づいてシステムを設計した。
The development team designed the system based on the specification document.
説明書 is a more general term for explanatory documents or instruction sheets, whereas 仕様書 often implies a more formal or technical document.
Compound of 仕様 (しよう, 'specification; way of doing') + 書 (しょ, 'document'). The word 仕様 itself derives from する (to do) + 様 (よう, 'way; manner').