HDTV, to which I referred as the requirement for high definition video output, was definitely Symbolics niche. I think HDTV-capable gear sized as individual workstations and not custom niche (even more niche than lisp machines, that is) systems only as pricy addons on SGIs and similar around ~1994, though some of the 1990~1994 Sun gear could do a lot there but AFAIK was geared more towards CAD clients than HDTV.

Unfortunately it seems like Explorer software is way less documented :/