noun which may take the genitive case particle 'no'
Used for documents or information that should not be disclosed outside a specific department or group. Often seen as a stamp or label on internal materials.
This document is confidential, so please do not show it to anyone outside the relevant parties.
I accidentally sent a restricted file to an outside party.
社外秘 means 'company confidential' and restricts disclosure outside the entire company, whereas 部外秘 restricts it outside a specific department.
Compound of 部 (department), 外 (outside), and 秘 (secret). The term likely originated in corporate or governmental settings to label internal documents.