Seal是“封印”，set the seal on是什么意思呢？请看对话：

    A: What do you think of the recent visit of the Australian foreign trade minister?

    B: His visit is significant because it will set the seal on the trade agreement between the two countries.

    A: I doubt it. With quickly changing exchange rates, nothing is certain at the moment.

    Note: This idiom, also seen in the form of “putting the seal on,” means “to make something certain or complete.” Traditionally, people would have their contracts written on paper when a “seal” or wax imprint sealed a document or letter, and showed an initial in the wax of the sender. Therefore, if you put a seal on something, it’s official and nothing would be changed further.