What a chimney cap is for
A chimney cap is the lid and screen over a flue opening. It keeps rain, leaves, and animals out of that passage while leaving enough open area for exhaust to leave. A missing cap after a storm is a common reason people search for replacement. A rusted screen that is matted shut is a different problem: it can block draft even though a lid is still present.
Cap, crown, and chase cover are three parts
On a masonry chimney the cap sits on or above the crown. On a framed chase the large sheet is a chase cover, and a smaller cap may still sit on the pipe that passes through it. Ordering a flue cap for a rusted chase lid leaves the water path open. If rust streaks run down siding, start with the chase cover page.
Salt air, storms, and idle fireplaces
Coastal air attacks screens, fasteners, and welds before the lid looks ruined. Inland humidity and pine needles load a screen that nobody cleans. A tropical storm can bend a lid or lift fasteners. Homes that go months without a fire still need that opening closed against rain and wildlife. The estimate should record the flue size, attachment surface, and whether the crown or chase top can hold the new cap.