Aṣl denotes a foundation, while farʿ denotes a branch or detailed question related to foundations. Belief gives religious practice its setting, while practical rulings clarify what is required in life.
Yet detailed legal questions cannot simply be derived from one creedal assertion without the methods of jurisprudence. Jurists examine texts, their implications, conditions and recognized procedures of inference. A disagreement over a subsidiary practice must not automatically be treated as a disagreement over the foundations of faith.
For example, believing that prayer is obligatory is distinct from a legal disagreement about some details of its performance; each requires the right kind of analysis.