There are two templates that involve items and their description.

Item description. Arises when the same model relates data and metadata. The item description template frequently occurs, but is visually less striking than other templates in this book—the template involves only two entity types and one relationship. You must exercise judgment in deciding when one entity type is metadata with regard to another.

Homomorphism. Maps between two Item Description templates and expresses an analogy. The description of item 1 is to item 1 as the description of item 2 is to item 2. The relationship between items connects occurrences. The relationship between descriptions constrains occurrences.