Please answer a question about the following article about Rule of law:

Despite wide use by politicians, judges and academics, the rule of law has been described as "an exceedingly elusive notion". Among modern legal theorists, one finds that at least two principal conceptions of the rule of law can be identified: a formalist or "thin" definition, and a substantive or "thick" definition; one occasionally encounters a third "functional" conception. Formalist definitions of the rule of law do not make a judgment about the "justness" of law itself, but define specific procedural attributes that a legal framework must have in order to be in compliance with the rule of law. Substantive conceptions of the rule of law go beyond this and include certain substantive rights that are said to be based on, or derived from, the rule of law.

What is the third and lesser referred to approach on defining the rule of law?
functional