In the docs it states that the possible status for the terminal checkout request are, PENDING , IN_PROGRESS , CANCEL_REQUESTED , CANCELED , COMPLETED. All of which I can simulate and that’s fine, but I can’t test what happens on a declined transaction, and wanted to know what status you get in that circumstance? Is it just cancelled? Thanks
The terminal simply asks the customer to retry on a declined attempt. Meaning the payment flow will never end on a declined attempt thus CANCELED would capture the declined event.
The status of a declined transaction will be IN_PROGRESS. This is because the Terminal will ask the customer to try to make the payment again. The customer can attempt the second payment with the card or they can try a different card. 
Brilliant, thanks for your help.