Refer Default settings
If you have common configurations for all your sites. Create those sites with Refer settings. This will improve the performance of the product and you can save your time by creating for every site.
Use case : If you have 100 sites and all those sites are having similar configuration then create that site with “Refer Default settings”. Now create your configurations under Default Settings.
Copy settings :
If a site is created with Copy settings, then the default settings will be copied and you can make changes on it.
Use Case : If you have 10 sites and most of the configurations are same as Default Settings and if you want to add one more criteria to it, then create the site with Copy Default settings. This will copy those configurations and after that you can add your changes to it.
If a site is created with Custom settings it will have its own settings.