This is PU letter. This letter is the key to get Visa.
In order to obtain the visa, the critical step is to get an invitation letter (PU).
Without this letter, it does not appear possible for a visa to be issued. You may have a residence permit, But it can not allow you to enter China. Based on feedback, to get the invite letter issued, you should be working at a company that has demonstratable economic impact in China (based on headcount, revenue, etc).
Note that some schools in first tier cities have successfully arranged PU invitation letters for staff - both Shanghai and Beijing have seen teachers return.
PU invitation letters are valid for 3 months from date of issue.
The only way to get this PU letter is let your boss to apply it in the foreign affair office.