Duvida sobre o web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	id="WebApp_ID" version="2.5">



	<display-name>fj21-agenda</display-name>


	<servlet>


		<servlet-name>AdicionaContato</servlet-name>
		<servlet-class>br.com.caelum.agenda.servlet.AdicionaContatoServlet</servlet-class>

	</servlet>
	

	<servlet-mapping>
	
		<servlet-name>AdicionaContato</servlet-name>
		<url-pattern>/adicionaContato</url-pattern>
		
	</servlet-mapping>

	<error-page>
		<exception-type>java.lang.Exception</exception-type>
		<location>/erro.html</location>
	</error-page>

	<error-page>
		<error-code>404</error-code>
		<location>/404.html</location>
	</error-page>




	<welcome-file-list>
		<welcome-file>index.html</welcome-file>
		<welcome-file>index.htm</welcome-file>
		<welcome-file>index.jsp</welcome-file>
		<welcome-file>default.html</welcome-file>
		<welcome-file>default.htm</welcome-file>
		<welcome-file>default.jsp</welcome-file>
	</welcome-file-list>

</web-app>

<error-page>
		<exception-type>java.lang.Exception</exception-type>
		<location>/erro.html</location>
	</error-page>

	<error-page>
		<error-code>404</error-code>
		<location>/404.html</location>
	</error-page>