The generated preset stores the full path of every media file. When vMix will read the files from a different place than the app (another drive letter, or another machine with a copy of the folders), fill in Custom Parent Folder on the home page.
It replaces only the part of the path before the playlist's source folder. The name of the folder that holds the nearby base preset is kept:
Base preset: C:\a\b\base file.vmix
Media file: C:\a\b\content\video.mp4
Custom: C:\custom\path
Generated: C:\custom\path\b\content\video.mp4
Leave it empty to keep the paths as they are on this computer.