SourceDir
Full path to the folder containing the original installation package. This variable may be used for paths that refer to files or folders relative to the installation package on the distribution medium. This is the same folder that acts as the root folder for any files that are distributed in plain (i.e., non-compressed) format. The actual files may be stored in this folder or any of its subfolders.
Usage
You can refer to it as <SourceDir>. Do not set this variable directly; it is set automatically when the installer runs. You can also set this variable by using the /s:sourcedir command line option.