Rated power vs power output
It seems that you are asking about the power used by a fetal doppler device such as a baby heart monitor.


  It says in the instruction book that the power output is 20 mW/ square centimeter ...


That tells you that the mechanical or sonic energy output by the device will be 20 mW/cm2. Therefore the total output power will be area &times; 20 mW where the area is measured in cm2. This figure will be useful in determining the amount of energy going through each unit area of the subject.


  ... but at the back of the machine it says: rated power less than 0,7 W.


That is the maximum power consumed by the device from the mains or battery power supply.

If the device does nothing else other than generate the ultrasonics then from these two figures you can calculate the efficiency of conversion of electrical power into sonic power: Efficiency = output power / input power. (Remember to multiply the specified output power/cm2 by the area first.) The efficiency is likely to be quite low.

If the device has other circuitry such as a display or an audio amplifier then that will consume some of the input power.