DEVDOCS-6392 | Updates to Orders domain developer documentation to im…

reference/orders.v2.oas2.yml

@@ -110,6 +110,10 @@ paths:
         * The values for cost_ex_tax, cost_inc_tax and cost_tax in the fees payload should reflect the tax rate associated with the tax_class_id. For a 10% tax rate, the difference between cost_inc_tax and cost_ex_tax should be 10%. If no tax_class_id is provided, the store's default "tax class for fee" will apply. Incorrect data may lead to issues in downstream operations like refunds.
 
         To learn more about creating or updating orders, see [Orders Overview](/docs/store-operations/orders).
+
+        <Callout type="warning">
+        If historical orders processed on another eCommerce platform are being migrated to BigCommerce, supply the `external_source` field with the code **M-MIG**. This code will exclude the historical orders from the store’s GMV/order count, which factors into pricing.
+        </Callout>
       summary: Update an Order
       tags:
         - Orders