Khass concerns a determinate assigned signification; thalathah, for example, means the fixed number three. ʿAmm focuses on how an expression includes multiple instances within the scope of its meaning.
Thalathah — a specific, fixed number.
Every student — an expression extending to all relevant students.
This comparison clarifies the concepts. Actual legal interpretation still requires examining the complete Arabic wording, its grammatical use and other relevant legal evidence.