The first one is any object, the second one is a list. It replaces the first element of the list by tho object. It replaces the rest of the list by its first argument and returns the new list without destroying the original list. The first is the key object which one seeks to associate in some way to an element of the association list which is the second argument. The first is the object to substitute, the second is the object which must be replaced by the first, the third is a list.
Substitution is made to all levels.
It is a more elementary function than SUB but its capabilities are less. When dealing with algebraic quantities, it is important to make sure that all objects involved in the function have either the prefix lisp or the standard quotient representation otherwise it will not properly work.