@WhichFolders is MIA in Designer Help

I would like to use this @whichfolders function in my custom databases. But I can’t find documentation on the function. I found this function being used in the Notes Mail template 8.5 in the All Documents view under the Folder Column.

Does anyone have any insight into this function?

If this isn’t available to my custom databases is there a solution to compute if a document is foldered and which folder or many folders it is in?

Subject: It is currently and internal feature

It is dependant on the mail navigator being opened and in focus. It relies on all of the folders in the navigator and their list of read and unread documents. This only gets accumulated at run time so this formula currently will not work in the back end.