In “lā fārisa illā ʿAliyy”, “fāris” is the attribute removed from other potential bearers and retained for Ali. The attribute is the restricted element, and Ali is the exclusive bearer.
The syntax reinforces this relation: “lā” negates the whole class, “fārisa” is its noun, and “illā ʿAliyy” singles out the remaining person.
“Restriction of an attribute” is not merely a question of whether a word is grammatically adjectival. It means limiting who possesses the relevant property, which differs from restricting a subject to one property.