AP Style is the Associaterd Press' style guide for American Journalism.  This should be understood. 

Writers in broadcasting, magazine publishing, marketing departments and public relations firms traditionally adopt and apply AP grammar and punctuation styles. Over the last 50 years, the AP Stylebook has become a leading style for non-journalistic publishers such as corporate marketing and public relations departments. Its simplified grammar, such as dropping the Oxford comma and using figures for all numbers above nine, saves scarce print and web space.