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

reference/orders.v2.oas2.yml

@@ -377,6 +377,10 @@ paths:
 
         The V2 Orders API will not trigger the typical [Order Email](https://support.bigcommerce.com/s/article/Customizing-Emails?language=en_US) when creating orders. To create an order that does trigger this email, you can instead [create a cart](/docs/rest-management/carts/carts-single#create-a-cart) and [convert that cart into an order](/docs/rest-management/checkouts/checkout-orders#create-an-order).
 
+        <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: Create an Order
       tags:
         - Orders