Description of "per-instance" licensing
An instance is one copy of the search.pl script (and all supporting files) in one folder on one web site. Each instance must comply with the license terms. If you register your script, each instance must be registered separately.
Consider a single physical web server computer. It can host multiple web sites. Each web site can contain multiple copies of the same script, in different folders. For example:
[server computer] e:/webroot/www.abc.org e:/webroot/www.123.org e:/webroot/www.xyzw.org e:/webroot/www.xyzw.org/english/search/search.pl e:/webroot/www.xyzw.org/french/search/search.pl
In this example, one server hosts three sites: abc.org, 123.org, and xyzw.org. Site xyzw.org has two instances of FDSE installed. Each of these two separate instances must comply with the license terms.
Note that you can have as many HTML search forms as you wish pointing back to that single instance. There is no limit on the number of HTML search forms that you place on your pages. Also, there are no restrictions or per-user costs based on the number of people using the search engine.
See also Description of "per-instance" licensing - Advanced for more information, including advanced examples and information on auditing and enforcement.
"Description of "per-instance" licensing" https://www.xav.com/scripts/search/help/1159.html