A chimney cap is called "un chapeau" (a hat) in French.
There are three chimneys on this, the south end of the house. One was for the central heating oil-fired boiler. That one is no longer in service with the new condensation boiler that vents through a new "hole" in the side of the house. The second is a twin to the stove hood chimney. I'm not sure why there are two in the kitchen; we think there may have been a coal burning heater in there next to the stove, but the second chimney is not in use. The third is the one connected to the current stove hood. Our wood-burning stove has its own capped chimney on the north end of the house.
We noticed last year, after a particularly hard rain event, that water seeped through the kitchen flue to the dry-wall where the stairs go up to the loft (close to the roof). We think that water got into the chimney, since they are open to the weather. So, the contractor sealed the two unused chimneys and added this cap to the third.