Briefing
Also called: letter
In a briefing, a client instructs what is expected of the executing party and what requirements an end result must meet. This explanation may be in writing or take the form of a meeting, such as a work session or meeting.
For assignments in business services, in particular marketing, communication and ict, it is customary to put the requirements and wishes on paper (digitally or otherwise) in a briefing. This briefing is usually provided when requesting bids or after approaching parties for a pitch, but in any case before the work and/or project starts.
A brief is derived and also referred to as a letter, which is English for short or concise.