The mark as shipped function is only available to the supplier, and will mark the SCR as shipped (no matter if you have filled in all the fields or not). There is not a confirmation screen or an undo, so if you are wanting to mark an order as shipped make sure that you really mean it. You should first edit the record so that you can include shipping and tracking numbers.
This action can be completed from the "updated", "pending", and "open" tabs in the supplier Quality dashboard.
Note: If you have a mark as shipped item that is showing on your "updated" tab, it is because it is letting you know that it has been marked as shipped. Your only available action will be to view. Once you view it will only show in the completed tab.
Keep Learning - Supplier Actions in Quality
Understanding the supplier quality dashboard
How do I respond to an SCR from the supplier side
Mark as shipped on an SCR <-- you are here