Search Engine Friendly Filter

The Search Engine Friendly Filter is an ISAPI Filter DLL, designed for use on Microsoft Windows NT 4.0 and higher, with Internet Information Server (IIS) version 4.0 or higher.

This means that it is loaded into the server or website process space where it is included in the chain of events as a request is received by the web site.
When a request is received that contains /SEFS/ and /SEFE/ in the directory structure, the filter converts it into a request with a querystring that then continues further down the chain until processed by IIS.