Patent trial fee is the fee that needs to be paid when the invention patent enters the trial stage. It is usually paid together with the trial request. After receiving the fee, the patent office will enter the trial according to your request, but it will be paid within three years from the date of application at the latest. However, three months before the expiration of the time limit, the Patent Office will inform you whether to request a trial. If it is not requested at the due date, it shall be deemed to have been withdrawn.
In any case, in principle, you can pay the actual trial fee after submitting the request. Don't delay too long, or it will be bad to forget the time.