The firestop radiation shield is used when passing through a second story floor, and at every floor above, to protect the floor joists where the chimney passes through the floor.
The firestop radiation shield is installed on the underside of the ceiling/floor framing, with the cylindrical "tube" pointing upward.
Example: in a 3 story home where the appliance is on the ground floor the support box is installed in the 1st floor ceiling, a radiation shield is installed at the 2nd and 3rd floor ceilings (where the chimney enters the attic space).
A firestop radiation shield is not installed where the chimney penetrates the roof.