The SXR formatter is a variant of the HTML formatter. However, as its focus is not so much documentation as code navigation, there are a number of important differences. Its default set of views is different, and instead of displaying listings of all identifiers on static html, it loads a database of (typed) identifiers and provides an interface to query them.
It is to be used with an http server, either a default http server such as apache in conjunction with the sxi.cgi script that is part of Synopsis, or by using the sxr-server program. The latter performs better, as the database is kept in-process, while in case of sxi.cgi it needs to be reloaded on each query.