Não sei se isso vai te ajudar, deve ser detalhe demais, mas dá uma olhada:
http://tomcat.apache.org/tomcat-5.5-doc/architecture/startup.html
http://tomcat.apache.org/tomcat-5.5-doc/architecture/startup/serverStartup.txt
...
The deployer will create a Digester for your context, this digester
will then invoke ContextConfig.start()
The ContextConfig.start() will process the default web.xml (conf/web.xml)
and then process the applications web.xml (WEB-INF/web.xml)
...
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