NextMemo
A memorandum or memo is written to convey information within a company. This use permits the memorandum to avoid much of the formality of a letter.
The style of the company will dictate the style of the memo. Formal organizations will expect memoranda to be written in formal tones avoiding the subjective, "I", "we", "you", in favor of the objective "it".
The direction of the memo also dictates the tone. Memos going upward (i.e.: to supervisors) will be more formal than those going downward (i.e.: to staff).
Why write a memorandum? To fix responsibility by establishing a written record of communications.