An important feature of many conceptual modeling grammars is the set of constraints they provide to allow analysts to show that real-world things may or may not possess a particular property. In the entity-relationship model, for example, the fact that a thing may not possess a property (the property is optional) can be represented by showing the minimum cardinality of a relationship or an attribute is zero (Batini, Ceri and Navathe 1992). Whether this practice should be followed, however, is a contentious issue because it may obfuscate the semantics of the real-world domain that is being modeled. An alternative approach is to eliminate optional properties from conceptual schema diagrams by using subtypes that have only mandatory properties (Weber and Zhang 1996). A problem with this approach, however, is that it often leads to more complex conceptual schema diagrams because they include more elements to represent the additional subtypes needed.
Bodart, Francois and Weber, Ron, "Optional Properties Versus Subtyping in Conceptual Modeling: A Theory and Empirical Test" (1996). ICIS 1996 Proceedings. 37.