First, the SPA page.  http://wiki.musicbrainz.org/Special_Purpose_Artist does have "Please note that you shouldn't create any new SpecialPurposeArtist all by yourself without prior discussion about it, usually on the style MailingList.", but we have no guidelines there as to how any new ones should be named.  A while back (1 or 2 years ago), several of us put most of the SPAs to the vote, to change them from "foo" (and the rest of the variations) to a standardized and distinguishable "[foo]").  I would suggest adding somewhere on this page a line saying something like "Special Purpose Artists should always be named in lower case with square brackets surrounding the name."

Next, Data Track and Data Track Style.  Do these need to be two different pages, rather than combining them into a single consise page?  Also, the line at the top of http://wiki.musicbrainz.org/Data_Track_Style - "If a DataTrack appears on a VariousArtists album, also enter "[data track]" as the artist."  This would seem to be a hold-over from when releases were only either VA or SA, rather than allowing SA releases with some tracks by different artists.  Is there any reason, on a single artist release, to not also have data tracks assigned to [data track] as the track artist, rather than just saving this for VA releases?

http://wiki.musicbrainz.org/Unknown_Artist is not too friendly, as Sami points out, and figuring out when it should actually be used isn't any better - "For details about when to use (and not use) [unknown], refer to UnknownArtistStyle." sounds fine, until you realize that http://wiki.musicbrainz.org/Unknown_Artist_Style is an entirely empty page (any contents were solely discussion, so the article itself is empty).

VA's not too great; http://wiki.musicbrainz.org/Various_Artists points to Release Artist Style ( http://wiki.musicbrainz.org/Release_Artist_Style ) for guidance as to when to use it...  great, but RAS has been a WIP for forever, and is not, in its current form, actually very useful.  For VA releases, it then also points you to look somewhere else - http://wiki.musicbrainz.org/Various_Artists_Release - which is a two sentence page, "A Release which consists of Tracks from various artists, none of which can be considered the PrimaryArtist of the release. Such a release is stored in the database with an ReleaseArtist of VariousArtists."  That's rather a long way to go to get from the VA page's "Releases where the TrackArtists are unrelated to each other (usually compilations), are attached to the artist "Various Artists"."  Could not Various_Artists_Release be merged into Various_Artists, with the (better) text from Various_Artists_Release replacing the (rather ambiguous) 1 line sentence at the top of the VA page?

http://wiki.musicbrainz.org/No_Artist is comparatively pretty good.  Defining it as a "credited performer of any discographic relevance" is not all that helpful though; [no artist] should remain for only those tracks or releases which truely have no artist - questions of whether someone is credited or not, or whether the performer(s) are of "discographic relevance" should have no place here...  (The performers on 90+% of wax cylinders from the 1880's through the mid 1890's had no performers listed; that shouldn't mean that those performers, when known, should not then still be used as the release & track artists for those...)  The example given here also is problematic - they directly conflict with the existance of the "[nature sounds]" SPA, using "(example: nature sounds releases)" as the sole use case for [no artist]...

Then there's http://wiki.musicbrainz.org/Special_Purpose_Artist ...  As Sami points out, some of the SPAs really should be merged/split away into other SPAs.  But as many times over the past years as I've read it, I still don't follow what "Others are used more informally: " is supposed to actually mean. 

The SPAs that the SPA page lists as "informal SPAs" are:
[anonymous]
[dialogue]
[gregorian chant]
[news report]
[spiritual]
[traditional]
Ethnic Music Compilations

The ones it lists above that ("formal SPAs"?) are

Various Artists
[data track]
[unknown]
[no artist]

I'd suggest that there is no need for this distinction.

It then lists "Incorrect collection" and "Collection" artists.  It's too bad we cannot somehow group all of these together, but minimally, I'd suggest that there is no need also for this distinction here - whether it's "Bollywood" or "Scary Sounds", they're both serving the same 'placeholder until we have better data' purpose, so there's no "incorrect" or "correct" here.

Apart from the (presumably "valid"?) collection artists, that leaves:

Bollywood
Disney
Data CD
Musical
MusicBrainz Test Artist
MusicBrainz Test Artist (Yet another test artist)
Soundtrack
Classical

plus there's the SPAs not listed (plus likely some that don't even use the [], so I can't find them...):

[Bysantine chant]
[Christmas music]
[Church Chimes]
[common chant]
[east african music]
[gregorian chant] & Antonio de Cabezón
[gregorian chant] & Francisco Guerrero
[gregorian chant] & Francisco Guerrero & Bricio Gaudi
[gregorian chant] & Philippe Rogier
[islamic chant]
[kiev chant]
[kiev-pechersk chant]
[language courses]
[musical]
[nature sounds]
[Sarum chant]
[television theme songs]
[znamenny chant]
Cenaclul Flacăra
Ella Fitzgerald & Chick Webb
Kimagure Orange Road
Orchestra

If I were reorganizing the artists and the SPA page lists, I'd suggest this:

"Valid" SPAs:

    * [anonymous] - The artist is unknown and unknowable.
    * [data track] - Rename it to [data], and merge in "Data CD".
    * [dialogue] - For soundtracks, some other valid uses as well.
    * [no artist] - There is no artist.
    * [unknown] - The artist is unknown, but possibly knowable. 
                        Merge in [traditional], Ethnic Music Compilations, Orchestra, and [east african music] for futher
                        cleanup (tracks can then individually be moved to [anonymous] oon a case by case basis).
    * Various Artists - Do we have documentation anywhere on when VA is valid anywhere outside of the release artist level?

Test Artists:
    * MusicBrainz Test Artist
    * MusicBrainz Test Artist (Yet another test artist)

Placeholder Artists (all subsets of [unknown]):
    * [religeous music] - merge in [Bysantine chant], [common chant], [gregorian chant], [islamic
                                                chant], [kiev chant], [kiev-pechersk chant], [Sarum chant], [spiritual]
                                                and [znamenny chant]
    * [classical music] - renamed from Classical
    * [musical theater] - merge in Musical and [musical]
    * [soundtrack] - merge in "Soundtrack", "Bollywood", and Kimagure Orange Road.
    * Ella Fitzgerald & Chick Webb - A "catchbin" artist, to snag newly added tracks and releases that would
                                                    otherwise end up dumped in (likely) the wrong places.
    * Plus all of the (valid) "collection artists" currently listed on the SPA page.

That leaves these, which arguably could be kept as special "subset" SPAs:
    * [Disney] - renamed from "Disney", subset placeholder SPA of the [soundtrack] SPA
                     (kept simply due to the number of unknown Disney-specific tracks)
    * [nature sounds] - subset of [no artist]
    * [language instruction] - Renamed from [language courses], subset of [no artist]
    * [news report] - subset of [no artist].
    * [television theme songs] - subset placeholder SPA of the [soundtrack] SPA.  Kept separate
                                             both because of the number of these tracks, and the higher chances
                                             of them being identified if kept grouped, rather than all just dumped into [soundtrack].

And these would simply go away with NGS:
    * [gregorian chant] & Antonio de Cabezón
    * [gregorian chant] & Francisco Guerrero
    * [gregorian chant] & Francisco Guerrero & Bricio Gaudi
    * [gregorian chant] & Philippe Rogier

That leaves Cenaclul Flacăra, [Christmas music] and [Church Chimes].  [Christmas music] could arguably be kept separate, merged into [religeous music], or simply merged into [unknown].  [Church Chimes] is one I actually created; it's a subset of [no artist], but there's possibly rationale for keeping it separate - these 1897-1902 releases are recordings of church chimes, released with either "Church Chimes" (most of them) or "Chimes" as the artist on the label.  And I'm not really sure just what we ought to do with Cenaclul Flacăra...

Brian