Here is a link where you can read more about web.config inheritance. While settings are inherited from the higher level of configuration file, child configuration files are always permitted to override them. Inheritance downward is useful for applying a unique settings for all applications on server but in certain situation a higher config file may want to prevent inheritance in child applications. This can be achieved by using the <location> element as follows.

<location path="." inheritInChildApplications="false">
<system.web>
...
...
</system.web>
</location>

The above code can prevent inheriting the <system.web> element in this case, into the child web application. Please note that this piece of code should be placed in the parent’s web.config file.