The Solr configuration files packaged with this module are provided in a way to make customizing as easy as possible. The “core files” with the base configuration for the Solr server are
solrconfig.xml. These should never be edited directly as they will have to be updated if future versions of the Search API Solr search module changes these files (though this shouldn't be the case too often).
The other files, however, only contain some default settings or only documentation, to help you customize your Solr server. These files will only rarely change, and when they do it should either be unnecessary to update your copies, or trivial to do so. Therefore, you can fill and edit them with custom settings specific to your site's needs. For the format of these files and what you can do with them, see the documentation comments included in them, or the official Solr wiki. The three
*_extra*.xml files are included into
solrconfig.xml when they are read, thus allowing you to easily add settings to them.
Remember: After changing any configuration, you will always have to restart your Solr server for the changes to take effect!