As is the case with other code access permissions, the IsolatedStorageFilePermission protects a resource from unauthorized access. In this case, the IsolatedStorageFilePermission encapsulates the ability to create and use isolated storage.
Application code is not required to request this permission because the runtime will grant whatever IsolatedStorageFilePermission is appropriate based on security policy. However, there are good reasons to request permissions that your application needs, including the IsolatedStorageFilePermission. For more information, see Requesting Permissions.
Demands for this permission are made when a file is created in isolated storage. A demand causes all callers in the call stack to be checked to ensure that each caller has at least the appropriate allowed usage. The quota of the code that opens or creates the store is checked when a store is opened.