How are Order Shipment Confirmation emails triggered?

In Commerce7, when the order's fulfillment status is updated to Fulfilled, an order the Order Shipment Confirmation transaction email will automatically trigger.
If you have the ShipCompliant integration set up, orders will be updated to Fulfilled automatically and this email will trigger to customers.
If you are manually adding tracking number and updating the status to Fulfilled, there is a checkbox to send the email or not. The check box to notify the customer will be unchecked by default. You can check it if you want the email to trigger.