This question is answered beautifully by Rainer Joswig on 'Stack Overflow' site.
Difference between `set`, `setq`, and `setf` in Common Lisp?
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What is the difference between "set", "setq", and "setf" in Common Lisp?
(set 'mat '(c4 d4 e4))
(setq mat '(c4 d4 e4))
(setf mat '(c4 d4 e4))