This is the file where all server-wide settings for IMP go. It is supposed to be generated through the web interface.
If DIMP and MIMP are installed, the IMP login page displays a drop down list that allows the user to pick the preferred webmail view. This decision is stored in a browser cookie, so that the selected view is selected by default the next time the user logs in.
With the configuration setting $conf[user][select_view] you can disable the drop down list.
There is no setting to make one of the views the default for user that have not logged in and picked a view yet. You can achieve the same with adding the following lines to config/conf.php though:
if (!isset($_COOKIE['default_imp_view'])) { $_COOKIE['default_imp_view'] = 'dimp'; }