The plugin named "server" is actually the core library (specifically, an object defined in the lib/snapwebsites.h.in header file.)
It is mandatory and offers basic functionality to the library, functionality that is actually implemented in the plugins. For example, once a child process receives a request from Apache, it parses the URI to make sure it is correct and then it calls the execute() signal implemented in the server. The path plugin listens for that signal to work on generating the output for the client.
The server being somewhat of an abstract plugin has no settings of its own at this point.