What a "brief" actually is, and why three disciplines agree
The word "brief" shows up as genuine jargon in at least three separate professional practices, each with its own real structural convention -- and all three converge on the same underlying job: fix the objective, the audience, and what "done" looks like, in writing, before the actual work begins.
Legal: the case brief
A case brief is "a short, concise summary of a court opinion, usually in outline form, containing all the essential elements and the main points of the court's opinion" -- prepared for classroom discussion or practice. The dominant structure is IRAC: Issue, Rule, Application, Conclusion. Variants like FIRAC (adds a separate Facts section), CREAC, or CRAC reorder the same core elements.
Marketing: the creative brief
A creative brief is the document an account team hands a creative team before production starts. Its core elements: project summary and objectives, target audience, key messages and brand guidelines, deliverables and formats, timeline and budget, success metrics, and an approval workflow -- so creative work has a measurable target, not an open-ended brainstorm.
Prompt engineering: briefing Claude on a task
Anthropic's own prompting guidance describes the identical shape applied to instructing a model: name the task explicitly, identify who the output is for, and set clear expectations for what "done" looks like before generation begins -- format, length, audience, and purpose stated up front rather than left for the model to infer.
Sources: IRAC Analysis (legal) · Asana: how to write a creative brief · Claude prompting best practices.