The “Diagnose” task is part of the "Restock processing" workflow and becomes available once the “Receive” step is completed. It allows your warehouse to check and confirm which products are actually in the box, what their condition is, and which actions should be available for those products.
This action becomes available in the actions button once the package is confirmed to be received.
The check screen is designed to give you all the information you need to book your return effectively.
After completion of the “Diagnose” task the "Process" (default) task becomes available.