For Z visa application to China, an invitation letter or fax from the department of the Chinese Government, together with an employment certificate issued by the relevant department or certificate of foreign expert. In addition, if you are going to work more than 6 months, a "Physical Examination Record For Foreigner" filled out and signed by a doctor shall be submitted.
For Business visa application, Chinese embassy there requires an invitation letter or fax from the Chinese company and a letter of introduction from your own company. But, applicants may not be granted visas as they wish, because visa officers are entitled to decide the visa type, duration,etc.