The Remaster PO file dialog box appears when you select the Remaster... command (the File menu).  You use this dialog box to choose options for remastering your message catalog.  For more information, refer to “Remastering your message catalog”

Update translation comments
Ordinarily, translation comments (to the right of the translation lines) are retained from your current file when you use the Remaster... command.  However, selecting this option causes remastering to also update these comments from the master file.