How does Sunspot modify Solr's schema.xml? Does it modify it at all?

Let me know if I am wrong, but I think solr only expects fields that are already mentioned in the schema.xml. So, if I have a field called 'title', I need to mention this in the schema.

There is no mentioning about modifying the schema.xml in the Sunspot's documentation. I am just wondering how Sunspot modifies schema.xml allowing custom fields to be entered to the index.

I also know that Sunspot uses RSolr to do things. So if there is a way to modify the schema and reload data from DB to Solr using RSolr, please let me know.