frontier is used figuratively whereas border is not *frontier
Related Translations:
比喻: 1.(譬喻; 一种修辞方式) metaphor or simile; analogy; figure of speech; allegory 短语和例子这只是一个比喻说法。 this is just a figure of speech2.(打比方) compare one thing to another 短语和例子原子结构时常被比喻为微型太阳系。 the atomic