If a child wants to research a given topic, they can not go to any other site than the ones their parents have listed. The Internet is severely limited as a research tool when using this program. This may be a good type of program for parents of younger students, but parents my find themselves always having to add new sites. Subtractive filtering programs run off a list of key words. The programs supply a list of words like violence, sex, pornography, etc.
Parents can add other words as they see fit.
Before going to these sites, the computer will check to see if any of these words are on the site. If any are found, the computer will not allow the child access to that site. It is called a subtractive program because sites with these words are subtracted from the child's choices. While having some limiting factors as a research tool, people who design questionable sites and want children to visit them can find ways around these programs. In some ways this may give the parents a false sense of security.