Final answer:
An operating system vendor that provides installation instructions for different platforms is offering an Installation Manual. This manual ensures users can install the OS securely across various platforms, and is specific to the installation process rather than being an SOP, a User Agreement, or a general cross-platform guide.
Step-by-step explanation:
A vendor who sells a particular operating system (OS) and publishes several sets of instructions for secure installation on different platforms is offering what's commonly known as a Installation Manual. These manuals are tailored to accommodate the unique requirements of various hardware and software environments, ensuring compatibility and security for the users. It is not a Standard Operating Procedure (SOP) as SOPs are generally broader guidelines for routine operations, nor is it a User Agreement, which is a legal document outlining the terms of use of software. Additionally, while it might contain cross-platform guidance, the documentation itself is best described as an Installation Manual, which provides step-by-step instructions.