FCE Writing questions always tell you who are you writing to. This is your Target Reader (TR). When you know your TR, you can decide in which register and style you need to write.
For example, in Part 1, the instructions always state, ‘Your teacher has asked you to write an essay’. You’re writing an essay for your teacher so you need to be serious and formal.
In Part 2, you need to think more carefully about your TR. A review or an article will be for the readers of a magazine or website. The most important thing about both text types is that your writing is interesting and informative. Informal to neutral register is usually required.
A letter or email will have different TRs. If your TR is a friend, write in a friendly, informal way. If the question asks you to write a letter of application, your TR is an employer so your register needs to be formal and your style serious, the same as a Part 1 essay.
For a report, read the question carefully so you know who your TR is, but it’s often the Principal of a college. A report is a formal type of text and requires an organised and informative style.
By thinking about your TR and selecting the best register and style, you increase your chances of getting a good score for Communicative Achievement.