Library service is, in essence, the retrieval and dissemination of embodied knowledge to individual members and groups in a community. Hence, the two essential parameters which affect the value of library services are: 

• Universe of Subjects.
• Universe of Readers; and

In order to achieve efficiency of services to readers , it has become imperative to adopt and develop such tools and techniques which would facilitate the classification of subjects embodied in documents and thus help in retrieval and service to the satisfaction of the laws of
library science. But, for this to happen, it is essential that the discipline of library science must keep developing itself to meet changes in the value of each of the parameters mentioned above.

In the succeeding sections, we shall consider the concern of library science with one of the parameters, namely, the Universe of Subjects (UoS).