1. Numerical three-, four-, or five-digit code located in the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code book and assigned to a patient’s medical condition, symptoms, or reason for the encounter as documented in the patient’s medical record. 2. Up to seven-digit code located in the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) . The code is assigned to a patient’s medical condition, symptoms, or reason for the encounter that is documented in the patient’s medical record (see Figure D-1, A ). 3. When the physician’s office or an outpatient hospital is billing, the primary diagnosis code(s) is inserted in Block 21 of the CMS-1500 insurance claim form. For inpatient hospital billing, the principal diagnosis code is inserted in Field 66 and subsequent diagnosis codes in Fields 67 through 75 of the UB-04 insurance claim form. Diagnostic codes.