SkyCat was developed at ESO as part of the VLT project and much documentation is available about the internals of how SkyCat (and consequently GAIA) are constructed. If you are interested in finding out about these then consult the postscript documents, which you can find from links on the SkyCat homepage15. The documents relevant to the version of SkyCat that GAIA is based on are also available at http://astro.dur.ac.uk/~pdraper/gaia/docs. People who would like to extend SkyCat or GAIA at the user interface level are recommended to read about “SkyCat plugins”.
15http://archive.eso.org/skycat/