A particular legal ruling addresses a specified matter, such as the obligation of prayer or the number of days involved in an expiation. A general principle governs the interpretation of numerous individual proofs, such as the force of an imperative or the authority of a report.
Fiqh asks: “What ruling applies to this concrete case?”
Uṣūl al-fiqh asks: “By what general evidentiary rules can such a ruling be established?”
The sciences are connected, not competing. A principle may guide many cases, yet its application still requires attention to the wording, context, conditions and any competing evidence. Simply knowing a broad rule does not license applying it mechanically to every situation that superficially resembles it.