English examiners are often strongly focused on communicative achievement, because it is the basis for the understanding. The higher is the grade of the exam the bigger is the importance.
You use an appropriate REGISTER for example, when you use a correct language without abbreviations or slang writing a letter to an important person or to an office or to a bank. Vice versa you can (or should) use slang or colloquial words, writing to a friend. In sum, it is the way you say a thing.
What about FORMAT. It is mainly how you put down your words on the sheet. For instance, in case of a report, you should start with the aim of the research, then explain the method used in the study, then the result in the following paragraph. You must end the report summarizing everything in just few words. If the target of the exam is for example an advertisement, its format must be absolutely different. I think it may begin with an important title, then with a slogan and a description.