ConfigurationAllowDefinition
Specifies the locations within the configuration-file hierarchy that can set or override the properties contained within a ConfigurationSection object.
Everywhere = 300
The ConfigurationSection can be defined anywhere.
MachineOnly = 0
The ConfigurationSection can be defined only in the Machine.config file.
MachineToApplication = 200
The ConfigurationSection can be defined in either the Machine.config file, the machine-level Web.config file found in the same directory as Machine.config, or the top-level application Web.config file found in the virtual-directory root, but not in subdirectories of a virtual root.
MachineToWebRoot = 100
The ConfigurationSection can be defined in either the Machine.config file or the machine-level Web.config file found in the same directory as Machine.config, but not in application Web.config files.