There is no single knowledge classification scheme on which everyone agrees. Michael Lesk speculates that while a single KOS would be advantageous, it is unlikely that such a system will ever be developed. Culture may constrain the knowledge classification scheme so that what is meaningful to one culture is not necessarily meaningful to another (Lesk 1997). Therefore, we live in a world of multiple, variant ways to organize knowledge.
Despite their diversity, KOSs have the following common characteristics that are critical to their use in organizing digital libraries.
• The KOS imposes a particular view of the world on a collection and the items in it.
• The same entity can be characterized in different ways, depending on the KOS that is used.
• There must be sufficient commonality between the concept expressed in a KOS and the real-world object to which that concept refers that a knowledgeable person could apply the system with reasonable reliability. Likewise, a person seeking relevant material by using a KOS must be able to connect his or her concept with its representation in the system.