The file left by the external vendor, containing detailed architecture information and code snippets, is best described as proprietary data. Proprietary data is information that is owned by a company and is essential to its competitive advantage. It includes sensitive business information such as trade secrets, intellectual property, and confidential data that should be protected from unauthorized access.

References = CompTIA Security+ SY0-701 study materials, particularly in the domain of data classification and protection​​.