The definition identifies usul al-fiqh as principles for deriving practical legal rulings from authoritative evidence. Its subject is the evidences insofar as they indicate practical rulings. Its purpose is to understand those rulings and develop the competence to derive them.
Asking how a general term covers its members is a question about an interpretive principle. Applying that principle to a particular text is an act of legal reasoning. The distinction between a rule of interpretation and its application is foundational.