This refers to the ability for a WinNet16 driver to distinguish special network directories by exporting the function WNetGetDirectoryType.
The types returned by this routine are fixed types known to the Windows 3.1 File Manager, with icons assigned by File Manager.
The MPR does not call WNetGetDirectoryType for directories that are on the network of a WinNet16 driver. Networks that require custom icons do this using Shell Extensions.