For this we need to change the "appSettings" section of the machine.config file. in this section we have an attribute "allowDefinition".
1.if it is set to "MachineOnly" then we cant override "appSettings" section in our web.config of the root folder.
2. if it is set to "MachineToApplication" then we can override in our root web application folder.
3. if it is omitted then we can override in multiple folders that is seperate web.config file for each folder.
Ali Akbar Mohammed