1. Code numbers inserted on claim forms by order, level, or rank. 2. In diagnostic coding for the outpatient setting, first list the primary diagnosis, which is the main reason for the encounter. The secondary diagnosis, listed subsequently, may contribute to the condition or define the need for a higher level of care but is not the underlying cause. Always code the underlying disease first, which is referred to as the etiology . In the inpatient setting, first list the principal diagnosis, which is obtained after the study that prompted the hospitalization. 3. In procedural coding, code the primary surgical procedure and then list secondary or adjunct procedures.