The server.xml file does not have a role from the servlet's point of view.
It is the main configuration file for the Tomcat application server and is not part of the servlet specification. It holds setting specific to Tomcat itself such as what ports to use, what connectors to use, the location and filename of any SSL certificates etc.
At one time it also held the <Context ..> entries for the individual applications (and still can) but those are usually configured in context fragment files.
The configuration file that does have a role from the servlet's point of view is the deployment descriptor (web.xml). To learn more about deployment descriptors, see the servlet specification (link in my signature).