<system.webServer>
    <rewrite>
        <rules>
            <rule name="AreaA">
                <match url=".*" />
                <action type="Rewrite" url="/SubA/{R:0}" />
                <conditions>
                    <add input="{HTTP_HOST}" pattern="suba.site.com" />
                    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                </conditions>
            </rule>
            <rule name="AreaB">
                <match url=".*" />
                <conditions>
                    <add input="{HTTP_HOST}" pattern="subb.site.com" />
                    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                </conditions>
                <action type="Rewrite" url="/SubB/{R:0}" />
            </rule>
        </rules>
    </rewrite>
</system.webServer>