Usually assigned as an examination question or homework
Purpose: confirm the student's knowledge
You want to display appropriate knowledge (indirect purpose); more convincing if you "pretend" you're interested in the question (the instructor or examiner can help!)
The audience is the instructor or grader; they want to see a compact, complete, well-organized essay (easy to grade) with required content
Goal is to get a grade; therefore satisfy the grader's needs
You may be better off with a clear, but clearly incomplete, essay than with an ambiguous, and possibly correct, essay.
Length: a few hundred characters (words)
Preparation time: 0 (test) to a week or so (homework) in advance
In an examination, unless you are experienced (or guessed the right problem in studying), you will need to allocate some of the examination time to planning. Make notes, an outline, on scratch paper, and check that you followed it when done.