Practical and subsidiary rulings concern the required, prohibited, valid or invalid manner of action. Foundational and creedal rulings concern the fundamental truths affirmed in religion. The first group is studied under religious law and rulings; the second is prominently named the science of divine unity and attributes.
Their subject matter was known in earlier generations, while systematic compilation became more necessary as cases, intellectual disputes and objections multiplied. That need encouraged the development of principles, ordered chapters, precise terms and formal arguments.
Unifying principle: identify the kind of question, select relevant evidence and reason in an orderly fashion. This prepares the ground for the next discussion of the evidence used in religious disciplines.