This option enables you to specify what options to export/expose to JS. Some options are required to be exposed for the default logic to work but if you have written a custom theme/interface it could be useful to restrict or allow more of config options.
Example of usage in the config.php file
$mcImageManagerConfig['upload.allow_export'] = "someitem";
Example of usage in the Web.Config file
<add key="upload.allow_export" value="someitem" />