A core class is a class that is written to store key code that a programmer uses many times within a program. Core classes can include many different pieces of code and typically help make an overall code structure more efficient by reusing code.
Techopedia explains Core Class
Core classes are often written as abstract classes, which allow for the definition and enforcement of various key protocols within the code. They can include display settings and other parameters, as well as parts of methods or functions, or other bits of code that interact to provide the intended results in multiple scenarios within the program. Programmers often include settings that help the core class to handle the system settings of a device, or manage functions based on conditions, such as whether a program is running.
In their written form, core classes resemble other parts of code. Creating strategic comments is especially important in core classes, so that others can assess the function and intent for these integral code modules. It is also important to define variables well for input and output, where core classes often pass variables to other parts of the program.