Candidates are required to write a letter of correspondence where they demonstrate their ability to communicate using English letter-writing conventions
You will need to choose and use the correct tone in your letter. Tone is the way you communicate with people that shows the kind of relationship you have with them. In letters it is clearly indicated by a proper salutation and closing and it should also be conveyed by your choice of words or phrasing as well as the kind and amount of details you include.
In addition to being given the situation, three bullet points will outline exactly what information you need to include in your letter. You might, for example, have to describe details, give reasons, express likes and dislikes, or make suggestions or recommendation
Different relationships require different levels of respect which is probably true in your language as well. IELTS letters are usually either formal or informal in tone.
Generally, if the letter is to friends, people you know well, or family, and the reason for writing is positive, the tone is informal. Letters to everyone else and for all complaints or negative messages, should be more formal.