If the :report argument to define-condition is used, a print function is defined that is called whenever the defined condition is printed while the value of *print-escape* is false. This function is called the condition reporter; the text which it outputs is called a report message.
When a condition is printed and *print-escape* is false, the condition reporter for the condition is invoked. Conditions are printed automatically by functions such as invoke-debugger, break, and warn.
When *print-escape* is true, the object should print in an abbreviated fashion according to the style of the implementation (e.g., by print-unreadable-object). It is not required that a condition can be recreated by reading its printed representation.
No function is provided for directly accessing or invoking condition reporters.
188.8.131.52 Recommended Style in Condition Reporting