In the past, it wasn't very easy to find this out, but it looks Apple added it to the OS X Server FAQ, located <a href="http://www.apple.com/macosx/server/faq/index.html" target="_blank">here</a>.
</font><blockquote><font size="1" face="Geneva, Verdana, Arial, sans-serif">quote:</font><hr /><font size="1" face="Geneva, Verdana, Arial, sans-serif">Mac OS X Server is available in 10-client and unlimited-client versions. Client restrictions apply to simultaneous Apple File Protocol connections only, primarily used by Apple file services, Macintosh Manager, and NetBoot. The client restrictions do not apply to Windows, UNIX, and Linux file services; email services; or web hosting services, such as Apache, QuickTime Streaming Server, and WebObjects.</font><hr /></blockquote><font size="1" face="Geneva, Verdana, Arial, sans-serif">