You can select Delete existing values for common keywords if you want to delete the values of Keyword Types that are common between the primary and related items before adding the values to the primary item. This option is only available when the action is configured to copy From This Item to Related Item.
Note:
When Selected Keywords is configured and Delete existing values for common keywords is selected, if all of the common keyword values are blank on the source item, the keyword values will not be deleted from the target item. For example, if the related item's common keyword type values are blank and are being copied to the primary item, the primary item's keyword values will not be deleted.