The value should be one of:
Eliminate all conditions.
Keep only the conditions that are in the class-precedence-list of
simple-condition error, and
serious-condition condition). This is useful for applications that use only
ignore-errors. It is equivalent to
:keep-conditions '(simple-error) :packages-to-remove-conditions '("common-lisp")
Keep all conditions.
A list of conditions to keep. For each condition, all the precedence list is kept.
See Deleting of condition classes.
LispWorks Delivery User Guide - 10 Aug 2017