When you’re installing your shell extension, you need to know which progid to hang it off of inside HKEY_CLASSES_ROOT. We’ll start with the title question and then move on to other predefined (but perhaps not well-known) progids.

• “Folder” is the progid for any shell folder. It could be a virtual folder (like Control Panel) or a file system folder (like C:\WINDOWS).
• “Directory” is the progid for file system folders. This is a subset of “Folder”.
• “*” is the progid for all files. Doesn’t matter what the extension is.
• “.” (that’s a single period) is the progid for files without any extension.
• “AllFileSystemObjects” is the union of “*” and “Directory”. It is the progid for all files and for file system directories.