What does the “numDecimalPlaces” in J48 classifier do in WEKA?
The short answer is: nothing. The numDecimalPlaces option, like debug and doNotCheckCapabilities, is part of the base class that all classifiers in WEKA inherit from. However, it is up to the implementation of the actual classifier to use this value to change the way the model is printed. There are no mentions of numDecimalPlaces anywhere in the source code for J48, which suggests to me that it does nothing in this case. 

It looks like the only classifier in WEKA that does anything with the value entered into numDecimalPlaces is LinearRegression.