The tags are basicaly additional information added before the start or after the end of the sound stream. With mp3tag, you can add custom tags, meaning tags wich have whatever name you want, and what ever value you want. However, mp3tag doesn't suppor aac id3v2 tags for a reason not even the creater of the programs is aware. When i asked him why not support such a feature, he said that this is just not happening. And i don't understant way, there are tagger out there that support this.

Anyway. For a program to recognize ALL the tags a sound file might have, he has to have a way to "scan" the begining and the end, for tag info. If it finds it, then it needs to sort out TAG NAME (which could be anything youre heart desires such as band composer, writer creator, beats per second, album artist, albumartist, etc) and then determine it's value.

This whole thing is messed up since there are a lot of different formats and version of tagging files.

Something universal has to be created, that work on each and every music file, in the same way (adding tag info at the beggining or at the end) and then every music player should have the ability to detect ALL tags and their values from a given sound file.

Since the adding of the tags is at the begiing or at the end, every sound file regardless of format should work with this universal tagging solution.
Just download mp3 tag and add your music files there. Then right click music file and hit extended tags.
A quick look on one of my downloaded mp3, reveals tags like ALBUM ARTIST COMMENT ENCODEDBY LANGUAGE TITLE TRACK WWW YEAR
As you can see there are a lot of extra tags that are just plain and simple useless.
It is the merit of the mp3tag to "see" these extended tags and their values, to bad it doesn't work for aac id3v2, the format i use for my music in N900
The thing is if we had via a program the ability to see modify and add whatever tags and values we like to whatever music file we have, and also players able to see all these tags, Nobody would stop you from adding custom tags like COMMENT to all your files and acctualy add a usefull comment to the file. And since you can see all the tags of a file on that hipotheticaly device that sees all tags, this wouldn't have been a useless thing to do.

Also, apart from this custom tags with custom fields, a LYRICS tags could be created, and the value it holds, could be in information structured in verses this way:
1)Time Information for instance 0.0-34.234
2)No words No lyrics, some sort of distinct sign
3)Time information 34.234-38.354
4)Verse 1
etc etc
The first time information means that that period of time from the start (0.0) until the 34second and 234ms is reached, there is no voice, and from the 34s234ms to 38.354 a voice is heard, and you have the following lyrics.

This way an "educated" lyrics reader could not only see the lyrics, but it can even display them as the song plays for maximum efectivity

If i's knew how to do this, i'd do this myself, but it takes lots of power and acceptance to create a standard.

And since no money is made out of this, i think we will never see things like this and we will always have trouble with tags and sound formats.
Especialy if someone else, decides to implement "their own" tag methods, such as apple for instance, wich would have the interest that the music they sell from their store to work only on their devices.