The enclosure notation represent that the writer of the letter has put other documents inside the envelope with their letter. The enclosure notation is used in a business letter. Business letter in general, are letters that are sent for corporate purposes often from one company to another, or from a company to its employees and customers, or vice versa.
The style of a business letter is highly dependent between the kinds of relationship that the concerned parties have among each other. Business letters are usually written for the sole purpose of keeping a written record to be used at later times for professional reasons.