It is basically a laboratory experimental teaching aid to demonstrate the effects of different R/C values on circuits,,
Usually a high quality resistor and capacitor substitution box. It provides a convenient selection of standard resistor and capacitor values available either separately or in series/parallel RC combinations. Typical applications include component substitution in servicing and circuit design applications, and establishing RC circuits for checking impedance, reactance, and frequency characteristics. The unit is also useful for classroom applications to demonstrate Ohm's Law, capacitive reactance, and other circuit effects caused by introducing various resistance and capacitance combinations.
It is not used solely in its own right but in conjunction with demo circuits that demonstrate the effects of different R C component values..
If you google R C decade box I am sure you will get many hits.
M.