In data modeling and knowledge organization we often find the need to connect item as subclasses or instances of each other. These two types of relationships are often confused, but why?
Where does subclass and instance come from? Object oriented modeling? Artificial Intelligence? Knowledge Organization? Plato? ...
Some influental sources should be identified.
See http://marciazeng.slis.kent.edu/Z3919/43hierarchy.htm on hierarchical relationships. Traditional thesauri don't differentiate Generic Relationships and Instance Relationships but just support Hierarchical Relationships.
Teorey et al (1986) extend the Entity-Relationship Model with hierachies to support generalization but the idea can be trace back further
Object Oriented Modeling was very influental starting in the 1990s.
See https://en.wikipedia.org/wiki/Class_%28knowledge_representation%29.
Where has the confusion been studied before?
There is no objective rule when to use subclass and when to use instance relationship because these categories are artificial. It all depends on...
Description logic does not work to describe reality.
Probably it has already been said by Kent (1978).