Luminescence Software/Metatogger

What's the difference between Artist, Album Artist, Artist (sort) and Album Artist (sort)?

*Example: *

Album info :
Album Artist = "The Beatles"
Album Artist (sort) = "Beatles, The"

Tracks info :
Artist = "John Lennon"
Artist (sort) = "Lennon, John"

So "*(sort)" fields are for sorting purposes only, and don't imply additional meaning, do they?

I don't think so.

Well, I can confirm this now, according to specifications of ID3v2.4 these fields do not imply additional meaning and should be used for sorting purposes instead of the main fields:
TSOA - The 'Album sort order' frame defines a string which should be used instead of the album name (TALB) for sorting purposes. E.g. an album named "A Soundtrack" might preferably be sorted as "Soundtrack".

Though I didn't find TSO2(used to store AlbumArtistSort) in specifications where did it come from?

Yes, the AlbumArtistSort tag doesn't officially exist in the ID3 v2.4 spec. So, the TSO2 frame is an unofficial frame, used by iTunes for example: http://id3.org/Developer%20Information

Well in this case I think that we should support these tags at the level of users tags and not as built-in tags...

This tag is commonly used and for consistency reason, I prefer to leave it with its other localisable "sort" brothers at the level of "uncommon" tags.

Understood, by the way, where did you got the info regarding which tags are common and which are not?

User requests, default tags in spec (Vorbis Comment, ID3 tags...), personal experience, and feeling... Not really an exact science.