I've read the definitions for plenum and riser, but they leave me wondering about actual application what is a plenum and what is not. And the context of the questions is should I use plenum rated cat6 and coax wire I want to run in the area.

My previous home had a single air return to the central HVAC in the basement, on the first and 2nd floor this air return was just a chamber with framing and drywall open to both floors and the basement with a metal duct sealed at the bottom to carry the air through the open basement to the hvac. There were return vents in the wall on the first and 2nd floors, for the air to be draw in from the floors to be returned to the HVAC.

This to me definitely is a plenum, air flow through this space. It could even be a riser, since its an open space between floors. If I were to run wires in this open space exposed to the air flowing through the HVAC they would have to be Plenum/Riser rated, cause any smoke produced by the wires failing could be circulated through the whole home quickly.

Now, my current home has an HVAC return from the first floor, that is drawn through a metal duct from beginning to end. Of course that metal duct is inside a space of framing and drywall surrounded by living space. I want to run wire through this space that the metal ducting is inside of, the wire is outside the metal ducting but within the same enclosed space the ducting is within. This space also has a floor sealed around the ducting, and a ceiling, the ducting goes to a wall vent on the same floor. So this wouldn't be a riser, since it truly 1 story tall, unless the opening for the duct that passes through the floor, and is sealed, changes that somehow.

So, I "think" but am not sure, this space is not a plenum nor a riser, and thus if I run wire through it (low voltage coax and twisted pair) it does not need to be plenum rated. Remember this is a metal duct within a closed space the size of a broom closet, the air only flows through the metal duct, the wire would be in the room, but not inside the duct, nor would it be close enough that it would even touch the duct. It has a ceiling and floor, but the duct runs through the floor. Am I correct, this space is not a plenum, nor riser and it is ok to use non-plenum rated low voltage wire?