Special Reel Set is a feature where a bonus round, such as free spins, is played not on the same reel configuration as the base game, but on a separate reel set specially designed just for the bonus.
The core of this feature is to fundamentally increase the value and anticipation of the bonus round by using a special reel set with a richer symbol configuration (e.g., removal of low-value symbols, addition of high-value symbols).