As internal messages, memorandums tend to be more informal and more direct than their letter counterparts.

作为内部信息，备忘录往往比信函更不正式、更直接。

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Further, letters are usually sent outside the organization, while memorandums are usually internal messages.

此外，信件通常发送到组织外部，而备忘录中通常是内部消息。

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