Digital Reference: An Overview
http://www.dlib.org/dlib/february03/02guest-editorial.html
  access restricted (by the publisher) to members/subscribers/customers only
  peer-reviewed

Silverstein, Joanne
D-Lib Magazine
2003

Abstract:

Digital reference refers to a network of expertise, human intermediation and resources placed at the disposal of users in an online environment. It employs automated tools wherever possible, allowing human experts to concentrate on 'hard questions'. But human expertise is expensive and hard to find. Automated tools are less expensive to incorporate into online services and sites, allowing digital libraries to use tools that were, until recently, the province of a small cadre of people.