SBFITSEXT began as a grass roots initiative in late 2002/early 2003 by astro-imagers who were concerned about lack of interoperability of FITS images between various mainstream imaging software vendors.
After some initial discussion, a draft proposal was offered by Matt Longmire, a software engineer with Santa Barbara Instruments Group. All of the major vendors at the time were involved in the discussion and review of the proposal. This eventually became "SBFITSEXT 1.0" on 19th March 2003 and was quickly adopted by the main vendors.
There was some discussion at the time as to whether it was appropriate to create a "standard within a standard" but the NASA FITS documentation explicitly allows this. Therefore SBFITSEXT is not part of the FITS standard, but forms a set of guidelines that vendors should adhere to if they want the images they produce to be usable in most image processing software. SBFITSEXT does not prevent vendors from using their own arbitrary keywords, but it would be wise to use these in addition to those documented in SBFITSEXT.
The document was originally hosted on SBIG's web server but this now returns a 404 Not Found error. Therefore we have provided a copy of the document here.