RDR (Rapid Deployment Run) is a special type of macOS installer that allows for rapid deployment of the operating system on multiple Macs. It's typically used by IT administrators and organizations to streamline the installation process.
Are you planning to install High Sierra on a or a virtual machine ? discussions.apple.com
The most secure method is to use Apple's official direct links or the App Store. Note that the App Store version usually downloads the final version (10.13.6), while specific version updates (like 10.13.5) are available as manual downloads.