1. ICD-9-CM procedure code for the most important medical service performed, usually for treatment, which is related to the chief diagnosis responsible for the admission of the patient to the hospital. If there are two procedures that are principal, then the one most related to the principal diagnosis should be the principal procedure. 2. When completing the Uniform Bill (UB-04) paper or electronic claim form, the principal procedure code should appear in Field 67. The electronic version requires an eight-character date listing year, month, and day (20XX0425).