Instead of the normal "Reply to All" behaviour, this option will make the 'Group Reply' toolbar button try to reply only to the mailing list through which you happened to receive the copy of the message to which you're replying.
'Erantzun guztiei' portaeraren ordez, aukera honek tresna-barrako 'Erantzun taldeari' botoiak soilik posta-zerrendari (erantzuten ari zaren mezuaren kopia jaso duzun zerrendari) erantzuten saiatzea eragiten du.