ICD-10 is the tenth revision of the International Classification of Diseases, the diagnostic coding system United States healthcare runs on in its clinical modification, ICD-10-CM. Every diagnosis a clinician records maps to a code, and the codes are hierarchical: they can express a condition generally or with precise detail about type, location, and severity.

Specificity is what makes coding matter beyond billing. A code that says a joint condition exists and a code that specifies which joint, which side, and what kind of degeneration are different statements about the same patient, and the more specific statement gives any downstream reader, including a benefits adjudicator, more to work with. Clinicians code to the specificity the clinical evidence supports, no further.

In a well-built medical record, coded diagnoses sit alongside narrative documentation of functional impact. The code establishes what the condition is; the documentation establishes what it does.