AppleShare: Extension vs Server & Drag Install

What is the difference between AppleShare (the extension) and AppleShare Server software? Why can't the AppleShare extension be drag installed?
The AppleShare extension enables a Macintosh to access network volumes and services through the Chooser. AppleShare Server software when installed on a Macintosh, allows network users to store, retrieve and share files and applications on a central, dedicated Macintosh server. The extension on an individual Macintosh allows the access the AppleShare Server running on a remote Macintosh.

The AppleShare extension cannot be drag installed because the Installer program must install System resources as well as the extension.
Published Date: Feb 19, 2012