If you're given a writing assignment, fulfilling it is your main purpose. If you don't understand the assignment, talk to your instructor. If you're asked to write a paper analyzing how an author's techniques contribute to his theme, and if you describe the theme thoroughly but don't discuss the techniques, then you've failed to fulfill the assignment. Or if in a psychology class you're asked to compare and contrast two theories about selective amnesia, and you write five pages on one of the theories and only half a page on the other, you probably haven't done what is required.
In many ways, writing on the job is similar to writing for a class assignment. For example, if you need to write a testing protocol for a new product, you should include such elements as a detailed description of the testing samples and the control group, the conditions of testing, materials and equipment, all the steps of the tests, relevant formulas and equations, and the methods to be used in evaluating results. If after the testing your manager asks for a summary of the results, you should provide it as clearly, honestly, and succinctly as you can. You may need to include a brief explanation of the tests, but you don't want to overdo the details. Whether you're writing in school or at work, make sure you understand your task, and then focus on achieving your purpose.