This is a federation of several independent SDLs in the network, organized
around a common theme, and coupled together on the network. A FDL
composes several autonomous SDLs that form a networked library with a
transparent user interface. The different SDLs are heterogeneous and are
connected via communication networks. The major challenge in the
construction and maintenance of a FDL is interoperability (since the different
repositories use different metadata formats and standards). Examples of FDLs
are the NCSTRL 9 and NDLTD 10.