Correct format of invitation letter for Z-type visa

I am Indian and now intend to go to china on Z-type. I have Original Invitation letter from Employer, Alien Employment Licence and Physical Examination Report. However there is a dispute on the information on Invitation letter due to which my application was refused. Chinese Consulate in Mumbai require that, Longest Duration of Stay to be mentioned as 000 in the invitation letter and Purpose of visit should be Employment.
Invitation Letter Authorization Agency in Jingjiang, Jiangsu is not allowing employer to insert Longest stay as 000 on the invitation letter. Please tell me what is the correct? Is there any supporting rule?