Newbie question from someone who hasn't looked at MAME in years

So I gather MAME is now MESS + arcade. When assembling a library with which to use the software, one finds a plethora of related directories, such as "a2600", "c64 _ flop", etc. The purpose of these directories is unambiguous. Less clear is what has become of the arcade component. When I go looking for a list of these directories and their corresponding intended contents, such as here, there is no reference to "arcade games" and no traditional "roms" directory to be seen. This confusion carries over to my search efforts to single out the arcade component of the required software as well.

Just seeking a clarification.