An enclosure is something that closes you in, like a pen or a cage. When a cobra disappeared from an enclosure at the Bronx Zoo, the entire Reptile House had to be shut down until the snake was found. An enclosure can also be something that's included in an envelope with a cover letter of some sort.
Under your name at the end of the letter, skip two lines. On the third line, write "Enclosure:" or "Enclosures:" if there are multiple documents. Skip a line after "Enclosures:" and then begin your list of enclosures. Put each enclosed document title on its own line.
If you need to note enclosures in a letter, leave a double space under your signature and salutation, then write “Enclosure.” To signify that your letter has more than 1 document enclosed, use the plural, “Enclosures” followed by a colon and the number of documents.
On a paper business letter, you often include the CC line underneath your signature, either above or below any enclosures line.