Which directory is commonly used to store additional application files that are not part of the OS installation?

The directory that is commonly used to store additional application files not part of the operating system installation is /opt. This directory is designated for add-on software packages that are not included in the default OS distribution. It is typically used for third-party applications, especially those that come with their own self-contained installations.

Using /opt allows for a clean separation between standard operating system files and additional software, which helps maintain organization and manage application files more effectively. Other directories, such as /usr, are utilized for user applications and utilities that are part of the system installations, while /srv is intended to hold data for services provided by the system. The /tmp directory, on the other hand, is used for temporary files and is not suited for storing software applications. This differentiation in purpose underscores why /opt is the correct choice for the context of additional application file storage.