When set to auto, the default setting, Setup will show a "The directory ... already exists. Would you like to install to that directory anyway?" message if the user selects a directory that already exists on the Select Destination Location wizard page, except when another version of the same application is already installed and the selected directory is the same as the previous one (only if UsePreviousAppDir is yes, the default setting).

When set to yes, Setup will always display the "Directory Exists" message when the user selects an existing directory.

When set to no, Setup will never display the "Directory Exists" message.