Letters, or anything else sent by post or courier, have enclosures – an enclosure is enclosed with a letter.
信函或者其他需要投递的事物也有附件，但在信函中要表示附件，则需要用enclosed一词。
E.g. Please sign the POA enclosed with this letter and return it to us as soon as possible.
例句：请在授权书(小编注：POA，即power of attorney的缩写)上签字，并附在回信中。另，请尽早回信。
