which points must be mentioned or which model of invitation letter should be applied by the company?

Kamran, in fact, you should obtain two invitation letters. One is from your business partner in China; the other is from a duly authorized unit in PRC, which can be acquired with the help of your business partner. The invitation letter from your business partner doesn’t need to be in a fixed format, but the following information should be covered: 1) information on the applicants, 2) information on itself. 3) and information on the planned visit. The invitation letter from Duly Authorized Unit is issued by the authorities, so you don’t need to concern about its format or model.