OpenROAD system classes are organized into an inheritance hierarchy. In this hierarchy, a class that is a child of another class is considered its subclass and a class that is the parent of another class is considered its superclass. Each subclass inherits the attributes and methods for all its superclasses.

The higher a class's position in the hierarchy, the more generalized the characteristics and functions that it defines. Classes lower in the hierarchy describe objects with specialized characteristics that function in a specific manner. For example, the characteristics and functions defined for the FieldObject system class apply to all fields and menu items, whereas the ButtonField system class defines a very specific object.

The inheritance hierarchy is useful in avoiding redundant definition of functions and characteristics. Attributes and methods are defined at the most general level possible and inherited by all subclasses.

For example, the ButtonField system class is a subclass of ScalarField. The complete list of attributes and methods available for ButtonField includes the attributes defined for ScalarField and its superclasses (ActiveField, FormField, FieldObject, and Object) and those defined specifically for ButtonField.