Return Identifiers are the strings that come on the Subject Line of an email and identifies it to be a bounced email.
You can add/remove/edit the subject string identifiers.
To add or remove an identifier, select the identifiers and then right click with the mouse. A menu will appear that allows you to delete the string or insert a new on. When a new search string is inserted, the default value is New Search String
To Edit a search string, click on the string itself and the edit mode is then enabled. Click on Save to save the search strings.
Click on Reset to default, it will clear the strings and reset to default strings. You will lose all custom settings.