Early in supercite's development, the supercite author, many of the major reader subsystem authors, and some supercite users got together and agreed upon a standard interface between all readers and any supercite-like package (of which supercite is the only known one :-). See section Hints to Reader Authors for more information on the details of this interface. You may want to also see section Thanks and History for details on how this interface was proposed and adopted.
Suffice to say that at the time of this writing (May 1991), only VM (all versions after 4.37) and MH-E (version 3.7, which is distributed with emacs 18.57) conform to this interface "out of the box." If you are connecting supercite to one of these two packages, you do not need overloading. All other reader packages must be modified to provide this interface. The file `sc-oloads.el' contains overloading routines which will allow you to connect supercite with any of the known major readers currently in existence (see section Overloading for Selected Readers). This includes: GNUS (versions 3.12 and 3.13), GNEWS, RMAIL and RNEWS (as distributed with emacs versions after and including 18.55), and PCMAIL. If you are using a reader package other than one of those mentioned above, contact the supercite mailing list about getting or writing overloading functions for that package (see section The Supercite Mailing List).