A written warning letter is an official document used by employers to inform an employee that they have done something wrong or failed to meet certain expectations. While it cannot be used as legally binding proof in court, it serves as a formal reminder of the standards which need to be met and can be used as evidence in future disputes. It is important for employers to issue this type of letter in order to make sure that their employees understand their obligations, and also to create a paper trail should any further action be necessary.