S1 Visa, Invitation Letter of Duly Authorized Entity or letter from Chinese university(original)

hi, i am from Pakistan. my wife wants to visit me in china. i m post graduation student in shandong. i was checking the embassy website, and the requirement. there they mentioned

(1) Invitation Letter of Duly Authorized Entity or letter from Chinese university(original)

what is this letter? where i can get that? can u tell me where i can get the sample? thank you so much..