Whenever you have a wood-burning stove installed in a fireplace, you will have a register plate above the stove blocking off the bottom of the chimney.
A closure plate is used when a chimney has no stainless steel liner. It closes off the chimney at its base, just above a wood-burning stove.
A register plate is used at the bottom of your chimney to close off the opening to the chimney, and must be sealed so it is gas-tight. A register plate will come with a pre-cut hole or access hatch.
It MUST be made of stainless steel (or other non-rusting metal like galvanized steel)