To emphasize the different aspects of this DL categorization, let us get into the
various DL types. In SDL and FDL, the items are electronically purchased or
fully digitized/scanned. These items are stored in the local repository (in SDL),
or in separate SDL repositories accessed using a network protocol (in FDL).
Each SDL holds a huge repository containing both the items and some metadata
structures to enable efficient retrieval. This material is updated every now and
then, in a process similar to the one in classical library. It is important to note
that composing a FDL out of SDLs requires interoperability capabilities, and
the use of a common protocol.