Referencing the name of a type (that is, a symbol) in code means that delivery cannot remove that type even if it is not used anywhere else. This is often seen in code using
to discriminate between types.
For example, if you have code like this:
(defun foo (x)
(cond ((typep x 'class1) ...)
((typep x 'class2) ...)
((subtypep x 'class1000) ...)))
then delivery would keep all of the classes
even if nothing else references these classes.
Possible solutions are described in Referencing types via methods and Referencing types via predicates.