TagLib::FileRef vs. TagLib::File

[Nouvelle Collection]

Hello,
I don't really understand the difference between TagLib::FileRef and
TagLib::File.

If I use TagLib::FileRef f("foo.mp3");, then I cannot use
f.tag()->removeFrames(...).

With TagLib::MPEG::File f("foo.mp3"), it seems possible to do
f.ID3v2Tag()->removeFrames.


Why is there 2 different things : TagLib::FileRef and TagLib::File
for the same task (i.e. working with a file) ?

When is it better to open a file with TagLib::MPEG::File f("foo.mp3")
and when is it better to use TagLib::FileRef f("foo.mp3")    (like in
the simple example in http://taglib.github.io/api/) ?

Thanks in advance.