Distinction between Doctrine of Severability & Severance

Yaar samajh nahi aaya, please help! Maine ye dono concepts padhe hain, lekin phir bhi confusion hai. Doctrine of Severability aur Severance dono hi constitutional amendments ko separate karna hote hain, lekin kya hain difference? Maine suna hai ki yahaan koi difference ni hai, phir bhi professor ne kaha hai ki ye do alag cheezein hain. Agar ek act mein koi unconstitutional part hai, to woh bura part separate kerke doosra part chalta rahega, yeh Doctrine of Severability hai. Lekin agar woh bura part bahut hi important hai, to uska separate karna chahiye? Ya kya yeh bhi aisa hi hota hai?