Rules Site Sync is used to copy your inventory rules down to child sites, this is down by copying the configuration.mof and sms_def.mof down to the sites. The tool will load the list of site servers based on the primary site you are connected to. You can select which sites to sync rules to.
NOTE: Although it is possible to have different rules at sites since inventory can be defined at the site level, it is highly recommended that you keep all rules the same from the top site down. As a best practice manage your rules from the central site then sync them down to all child sites.