[Reviewer Note by Barmar: This description is hard to follow.]
Methods for update-instance-for-redefined-class may be defined to specify actions to be taken when an instance is updated. If only after methods for update-instance-for-redefined-class are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of update-instance-for-redefined-class. Because no initialization arguments are passed to update-instance-for-redefined-class when it is called by the system, the initialization forms for slots that are filled by before methods for update-instance-for-redefined-class will not be evaluated by shared-initialize.
Methods for shared-initialize may be defined to customize class redefinition. For more information, see Shared-Initialize.