Attributes Attributes are properties used to describe an entity. Attribute names are unique throughout an IDEF1X model, and the meaning of the names must be consistent. Each use has a range of meaningful values, and thus, the entity must be distinctly named. Each attribute is owned by exactly one entity. Other occurrences of the social security number attributes would be inherited across relations.
Rules enforce creating proper models.
In a situation where it seems that a rule cannot hold, the model is likely wrong. Keys A key is a group of attributes that uniquely identify an entity instance. There are primary and alternate keys. Every entity has exactly one primary key displayed above the horizontal line in the entity box. Entities can have alternate keys that also uniquely identify the entity, but are not used for describing relationships with other entities.