These functions destructively alter the form of a list. rplaca modifies the car of a list, effectively replacing the first element. rplacd modifies the cdr of a list, replacing the entire tail of the list. These functions have no meaning for non-lists. To entirely remove the tail of a list, replace the cdr of the list with nil.
Gamma> x = list(1,2,3,4);
(1 2 3 4)
Gamma> rplaca(x,0);
0
Gamma> x;
(0 2 3 4)
Gamma> rplacd(x,list(7,8,9,10,11,12));
(7 8 9 10 11 12)
Gamma> x;
(0 7 8 9 10 11 12)
Gamma>