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.
In law specifically, a memorandum is a record of the terms of a transaction or contract, such as a memorandum of understanding, memorandum of agreement, or memorandum of association.
Dean Acheson famously quipped that "A memorandum is not written to inform the reader but to protect the writer".
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."