A statement of the form "noun A なら predicate X" says that the predicate X applies only to A and is not more generally valid.  The main ideas of a なら sentence, in other words, are "limitation" and "contrast".

    Ex:  Q:ブラジルに行ったことがありますか。
　            Have you ever been to Brazil?
         A:メキシコなら行ったことがありますが、ブラジルは行ったことがありません。
                              I've been to Mexico, but never been to Brazil