Hi, Mr. Biland, as far as I know, invitation letter is not necessary for Z work visa application. What you need to provide for the Z visa application is a Visa Notification Form issued by Chinese authorized unit and a Working Permits, such as: Working Permit issued to foreign experts by The State Administration of Foreign Experts Affairs, Alien Employment License of the People's Republic of China issued by Ministry of Labor and Social Security, Letter of Invitation for Foreigners Engaged in Offshore Petroleum Operations in the People's Republic of China, etc.