Why do the library files' names start with AC _AP_ AR_

Hey guys.There are many library files in the libraries directory. Their names start with AP_, AC_, and AR_. What are the different meanings of these beginnings?

AP is for ArduPilot. It is a library for all vehicles
AC is for ArduCopter. It was originally for Copter type only
AR is for ArduRover. It is for rover type

@khancyr Thanks, so this is a rough division? Like the PID library starts with AC_ and the WPNav library starts with AR_, but I guess both the libraries are used for ArduPlane.