Dr. Clarks real website is http://www.drclark.net and she does not sell or endorse ANY Zappers at all. The design in her book was made to be simple and to kill ALL parasites in the 7x3 sessions.
"... Any positively offset [DC] frequency kills all bacteria, viruses and parasites simultaneously given sufficient voltage (5 to 10 volts), duration (seven minutes), and frequency (anything from 10 Hz to 500'000 Hz)."
Others have messed with it, but if it falls within the range stated, it will kill all of the bugs, no matter if it's higher or lower or targeted etc. Don't worry about getting one for certain things, that's just a marketing blow up and it doesn't make any difference. The main thing is to have a good battery or a power supply, otherwise they all work the same.