OSX Lion Server Security If lion server runs on osx lion, what is accessible from the "outside"?

If I open a port and connect it to the ip of the lion server, will that traffic be intercepted by the server app and limiting the scope of the access to what the server app allows access too?

Will my entire file directory be available to the outside if I open the port? E.g. the lion OS running has some other apps running and files stored on it, will this be accessible from the outside? Or does the server app intercept the incoming requests and, depending on what has been made available as a service through the server app, that is only what the outside can access?
OSX Lion isn't an app as such but a collection of additional services and applications running on your Mac. Accessibility from outside is the same as for a plain vanilla OSX installation.

Any port you open allows access from outside, but access is always (as long as there are no security bugs) restricted to the service provided. So if you enable Web Sharing, access to the local web server is possible (including ~user/Sites) but not (unless you reconfigured the web server in an insane way) to your whole filesystem.