"F" Visa is issued to an alien who is invited to China for a visit, an investigation, a lecture, to do business, scientific-technological and culture exchanges, short-term advanced studies or internship for a period of no more than six months.
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Z Visa is issued to an alien who comes to China for a post or employment, and his/her accompanying family members.
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Tourist / Family Visit (L) Visa

Business Visit (F) Visa

Employment/Work (Z) Visa

Study (X) Visa

Transit (G) Visa

Resident (D) Visa

You need a Z visa to teach English in China. Presently your employer is offering a F visa (buisness) valid only for 6 months.