Final answer:
In medical billing on CMS-1500 forms, each procedure or service in Box 24D is linked to a diagnosis code from Box 21, which contains the ICD codes for the patient's diagnosis.
Step-by-step explanation:
The question relates to medical billing and coding, specifically pertaining to the process of linking diagnostic codes to medical services and procedures provided to a patient. When filling out medical claim forms, such as the CMS-1500, the correct diagnosis code for each procedure or service is entered in Box 24D, and is referenced from Box 21. Box 21 is where the International Classification of Diseases (ICD) codes are listed that correspond to the patient's diagnosis. Therefore, for each procedure or service in Box 24D, the correlating diagnosis number comes from Box 21.