From the main menu choose Settings → File permissions groups
This setting is used when installing new sites/modules. If set, after installation of the files of given module their permissions and group are changed to this setting. This is necessary because when a file is created via TCC it is initially owned by the user used by Apache and you can't edit it (usually). In future versions of TCC this setting will be expanded to have separate permissions for directories and files (i.e. to be able to set 775 for directories and 664 for files).