Scale and polish is done when your teeth require a thorough clean to remove plaque and tartar/calculus build-up. Calculus or tartar is basically hardened or calcified plaque (hardened over time by the minerals in our saliva) that was not thoroughly removed by your daily brushing.
Scaling is needed to remove the hard tartar on your teeth using ultrasonic vibrations, which cannot be removed by simply brushing your teeth. (for daily brushing we always recommend an extra soft toothbrush).
