What is the difference between dental cleaning and dental scaling?

My health insurance through work covers "8 units of scaling" per year. What does this mean?
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.HELLO, A CLEANING, AKA PROPHYLAXIS, IS MUCH LESS INVOLVED THAN A SCALING, WHERE ANESTHETIC MAY BE GIVEN AND IS MORE EXPENSIVE. A UNIT FOR SCALING NORMALLY MEANS 1 QUADRANT, THE MOUTH HAS 4, SO THIS MEANS 2 FULL-MOUTH SCALING EACH YEAR -- IF 1 UNIT=1 1QUADRANT.
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