Final answer:
A four-digit ICD-9-CM diagnosis code is referred to as a subcategory code. It provides more specific information about the diagnosis compared to the three-digit category codes.
Step-by-step explanation:
A four-digit ICD-9-CM diagnosis code is referred to as a subcategory code. The International Classification of Diseases (ICD) is used to classify diseases and monitor morbidity and mortality. The four-digit codes in the ICD-9-CM system provide more specific information about the diagnosis compared to the three-digit category codes.