XF 2.1 Q: Xenforo architecture

Where does the terminology of Xenforo architecture come from?

For example, apparently a "Service" is a specialized object that is supposed to perform a single action. And you are supposed to use "XF:User\UserGroupChange" service to assign user to a group. Why is it called a Service?

Or the idea to extend classes via proxies?

Is there a book or some other definitive source that uses the same terms and approaches and explains them?

Yeah, I know about the manual

I am more interested about why it is done this way. A historical perspective? Some previous application that inspired it? Or at least something like "It is a standard MVC lingo, you should read this book, it explains it". Or is it completely home grown?

Services aren't unique to XenForo. In the computer world, any routine that is globally used to do something is usually called a service. Especially in object oriented programs. In XenForo, that includes sending emails, conversatons, and quite a few other things.