It means it is restricted to horses that were qualified for novice chases last season, or are still novices now. So if you won a chase in the 2013/14 season, or earlier, you can’t run this race.
Designed to gives horses that lack experience the chance to race against others of the same type before they move up to races like the King George. There used to be a lot more of this class of race in the program, including an entire series of races through the season leading to a final staged in January over 2M 4F.