Storing URL rewrite mappings in a separate file
<system.webServer>
  <rewrite>
    <rewriteMaps configSource="rewritemaps.config" />
    <rules configSource="rewriteRules.config" />
  </rewrite>
</system.webServer>

In addition you can move quite a few configuration sections to their own files:

<appSettings configSource="appSettings.config" />
<connectionStrings configSource="connectionStrings.config"/>
<pages configSource="pages.config"/>

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