For ambulatory surgery, assign an icd-10-cm code to the diagnosis for which the surgery was performed; if the postoperative diagnosis is different from the preoperative diagnosis when the diagnosis is confirmed, assign a code to the

Post operative diagnosis.

Explanation:

ICD-10-CM are the diagnosis codes used to determine the disorder, symptoms and poisoning of the disease. These codes are used as a tool for to describe the disease in the form of codes instead of written documents.

Ambulatory surgeries requires the  post operative diagnosis because these diagnosis are more definitive than pre operative diagnosis.  Post operative diagnosis code is more reliable and helpful for the ambulatory surgery of the patient.

Thus, the answer is post operative diagnosis.