What is A handpiece with <12,000 rpm can be used for which of the following situations? and explain in detail?
Three speed ranges are generally recognized: low or slow speeds(<12,000 rpm), medium or intermediate speeds (12,000–200,000 rpm), and high or ultrahigh speeds (>200,000 rpm). The terms low speed, medium speed and high speed are used preferentially.
The low-speed range is used for cleaning teeth, caries excavation and finishing and polishing procedures. At low speeds, tactile sensation is better and generally overheating of cut surfaces is less likely. The availability of a low-speed option provides a valuable adjunct for many dental procedures.
At high rpm, the surface speed needed for efficient cutting may be attained with smaller and more versatile cutting instruments. This speed is used for tooth preparation and removing old restorations. 

Ref: Sturdevant's art and science of operative dentistry. 4th edition page 321