The charge letter is what an accused student would receive at a certain point during the university disciplinary process. It would generally come from the office that's investigating or adjudicating the case. The charge letter, it may come immediately upon the student's first notification that they're involved in a case. Some schools actually will investigate the case first, where a student would be notified that they're subject to an investigation before they're actually charged.
If there's eventually, through the course of the investigation enough information to determine that there's enough to warrant a charge against the student, then at that later time, the student would receive the charge letter. It does depend on the particular school involved. It also would depend on the particular offense involved, be it say a Title IX offense, a sexual misconduct that is, academic misconduct, a general disciplinary issue. School's vary, it's critical to have an experienced attorney advisor helping navigate the process so that your rights and interests are protected.