5.6.4 Setting an Identifying Element
An identifying element is used to identify a specific occurrence in the entity. For example, a Customer entity may have an identifying attribute of Customer Code. The Customer Code is used to uniquely identify a Customer.
The identifying element is set by selecting the element to be used as the identifying element and clicking the 
toolbar button. You can change the identifying element by selecting another element and pressing the button. If you use the button with the current identifying element selected, it will be changed back to a standard element for the entity.
Note that more than one element may be necessary to identify an occurrence within an entity. In these cases, Parent/Child relationships supply inherited elements to make up the complete set of identifying elements. Inherited identifiers are denoted by a gray key symbol immediately to their left.
It is very important to remember to add identifying elements if you have relationships between entities. If entities are used in relationships and do not have identifying elements, the model build will fail with validation error messages.
If the entity is built into a database file, the identifying element will be used as part of the key to the file. The complete file key may be based on inherited elements as well as the identifying element.
Almost all entities will have an identifying element. If an entity has two or more parents, it does not require an identifying element.
Note that identifying elements are not permitted in Conceptual or Variant entities.