When you write, you need to think about your register. In other words, how formal does your writing need to be? In addition, you need to think about your style as well as register. The two go together.
For example, you always write a Question 1 essay for your teacher, so your register needs to be formal and your style needs to be serious and informative.
In Part 2, different styles are required. An email to a friend requires informal register and a friendly style. A job application? Formal register and businesslike, confident (but not arrogant) style. A magazine article? Make sure your style is entertaining and interesting because you want your reader to enjoy your text.
Selecting the correct register and style will help you to get a good score in Communicative Achievement.