Should I apply for F visa or X visa if I go to southeast university, nanjing for three months study?

Dear friends

I am from iraq. I got the acceptance notification from southeast university, nanjing for three months due to I have a project in nano field . In the admission copy was written that (you can apply for student (F) visa). but in the below of the admission was written that ( Be sure to enter China with an x visa, otherwise you will responsible for all the possible consequences. what is that? and why? How can I get it in iraq.
Thanks