Thank you for your question.
The register plate is fitted into the top of the fireplace chamber to block off the bottom of the chimney. It serves to hide all the liner and pipe work from view, to stop any debris from falling onto the stove and to cut down on draughts that may come from the chimney.
The flue liner takes the smoke exhaust from the stove to the top of the chimney and the pot hanger holds the liner in place, stops rain from getting into the liner and deters birds from finding a nice warm spot in your chimney. All do distinct jobs and we recommend you use them together to get the most from your stove.
Hope this helps?