Audit Entry with Event named Book

Hi,

I see an audit entry with Event name of Book. My understanding is that it is typically a book request for appointment entity. I dont have any plugin or workflow code that raises BookRequest. It should happen through outlook. I tried creating an appointment in outllook, set Regarding and track that appointment in CRM. All it does is create and update events. Can anyone tell me on when aBook request is raised from outlook?

Thanks in advance.

vijay