A memorandum or 'memo" is a note or communication that aids the memory by recording events or observations on a topic, such as may be used in a business office. The plural form is either memoranda or memorandums''. A memorandum may have any format, or it may have a format specific to an office or institution.
The formats of briefing notes vary widely. They could be memos, reports, letters or binders. They could be one page long or a hundred. If the user is a cabinet minister or a senior executive, the format might be rigidly defined and limited to one or two pages. If the user is a colleague, the format might be entirely up to the writer. At its most basic level, a briefing note could be a handwritten note to one's supervisor, saying: "Mary, could I ask you to sign the attached requisition? I have checked our budget, and we have the funds for everything we need to order."