Hello. I am from Pakistan. I will be presenting a paper in a conference which will held in Nanjing, China. I have an invitation letter from conference but the above article says that "Visa Notification Form issued by an authorized governmental office or organization in China prescribing the visa validity, number of entries and stay period, or an official invitation letter from the authorized host company, institute, meeting or exhibition organizers in China." is required what is it and isnt the invitation letter from conference is enough?