This is a frequency source system that is controlled by a digital (microprocessor/microcontroller) system. This circuit produces  a square wave signal with adjustable frequencies. Here is the block diagram of digitally controlled frequency source system:

DIGITALLY CONTROLLED FREQUENCY SOURCE circuit schematic

This frequencies are controlled by microcontroller register, counter or thumb-wheel switches. This system can be used in many applications like numerically-controlled machine tools and computer-controlled test equipment.