FLAT BASE - kettlebells need to be created with a flat base for easy storage and use during floor workouts, kettlebells without flat bases will roll around which makes it hard to store and use during movements like pushups, rows and renegade rows 
GRAVITY CAST - this ensures precise weight, you need to be confident you are training with the correct weights. Kettlebells which are gravity case are more weighted more precisely, kettlebells which are not gravity cast have imperfec