Hidden folders are generic folders that clients create within the root folder of the message store rather than the root folder of an IPM subtree. Because these folders are not placed in an IPM subtree, they are generally hidden from the user's view by the message store provider. Hidden folders typically hold information relevant to the message store but irrelevant to the user. Clients create hidden folders to store, for example, additional information to be saved with the rest of the folder hierarchy.
MAPI expects all clients to be able to display, create, modify, and delete folders in an IPM subtree. Support for working with folders in other trees is considered optional. However, all message stores that can be used as the default store and that can send and receive messages should fully support hidden folders.