invitation letter requiremt for multiple entry visa to china?

Hi
I am from Iran and want to apply for one year multiple entry visa (business visa) to china. The company I work with in china has asked me to tell them what type of invitaiton letter the should send me and what they should include in the letter? Would you be kind to tell me how to anser them. The embassy does not seem to give a very clear answer. Please also tell me whta documents do I need
regards
Hossein