An attachment or enclosure notation in a letter indicates that the sender has included additional documents for review in the envelope. If singular is used, such as attachment or enclosure, then only one document is included.
Plural forms, such as attachments or enclosures typically have a numeral following the word to indicate how many attachments are included in the envelope or package. The word attachment is typically reserved for electronic letters included in an email, whereas the word enclosure is used when sending a letter or package via postal mail. The notation "cc" includes a list of the people who also received a copy of the letter.