Can I get a duration of stay for 18 days if the longest stay in my invitation letter says 15?

I have to attend a crafts exhibition in China. The organizer has sent me an invitation letter. The "Longest Stay" column reads 15. Can I get one for 18 days? I have a valid air ticket for my travel period of 18 days. The "Validity of Visa" column in the letter reads 6. What does that mean? Can I visit places of tourist interest after the exhibition? Thank you. Mrs. Malavika Neogi Chatterjee.