How customers get tracking number?
The Shopify shipping notification email template contains a tracking number/track delivery button and link. By default, the tracking link would be navigating to Courier tracking page.
Essentially, if there's no tracking number provided when you're fulfilling an order, there won't be a link in the email to track the item. Below are some screenshots for you to show the two emails that are sent once an item is shipped.
Screenshot 1 shows the email without tracking:
Screenshot 2 shows the email with tracking:
Keep in mind, you're in total control of the content of the notifications your customers are sent. We have a great guide that walks you through the different notifications you can edit here: https://bit.ly/2QGcTQv.