I've always thought about like this.
R&B Legends: Mary, Toni, Robert, Brandy, Usher - the commercial success and industry respect speaks for itself.
Icons: Faith, Tyrese, Fantasia, Tank, Dru Hill, Joe - people that had a lot of success back in the day and still book an Urban AC hit here and there. Revered for their talents and still tour a lot.
then you have your KeKe Wyatts, Avants, Monifah's etc