This may be regional terminology thing but for us the skirt has only 3 sides and an open bottom and top and goes against the wall under or above a panel. We would typically use these for coming up out of a raised floor to conceal cables that enter the bottom of the panel. Same would apply with cables entering from above.
The Extension is just that, it becomes part of the panel and can have conductors run through it just like within the panel. IMO a skirt does not require any listing because electrically it does nothing and is merely cosmetic.