I did not find the relevant info in the style guideline.
There are cases when physical releases have additional artists credited for particular tracks, but not mentioned in its titles. I mean significant contribution, like co-writer or additional vocal. Is this still considered a “featured” artist to be credited (instead of just adding a relationship)?
Assuming you mean stuff like “X: guitar on track Y”, then no. Only artists listed with things like “feat.” or “with” or “&” should be in the link phrase - the rest is definitely relationship only. Do you have an image of the exact case you have in mind?
Yes, please see the credits on the artwork at https://www.discogs.com/release/7013617
Definitely relationships only here 
So this is solely based on the track title as present on the artwork? Because in this particular case some tracks are specifically credited to more than one author.
Not exactly on the track title, but on the tracklist in general. Most pieces of music are not made by one artist only, but we’re trying to store who gets the main credit (which in this case is the album artist only, since nobody else gets a front cover or tracklist mention), with the stated facts of who did what listed as relationships.
Tracklist, as present on the artwork, is sometimes subject to graphical design. Thus, frankly speaking, I find the reasoning above shaky.
The release I referred to serves as a good example. How else “UNEQUAL written by Holly Herndon and Colin Self” should be understood other than that the track has two authors?
The specific intent of the track artist field is to follow the printed artists. All the juicy info is in the relationships.
It should be understood (and added) as “this is a recording of a composition (work) with two writers”, without changing the actual track credit. Otherwise, for example, every track in an album by Britney Spears should be credited to who knows how many people who actually wrote the song, but it’s clear that the intention is to credit them to Britney Spears.
Dragging here a singer like Britney Spears is simply reductio ad absurdum. The release in question is purely synthesized music (except the samples of course). People involved in its creation are explicitly mentioned in the credits (including their roles). This is the intent, if you prefer to appeal to it.
I can easily understand the rule “do it as on the tracklist”. No questions. I can also understand “it was decided to do it that way, period”. What I do not understand is what is that rule that can be applied consistently, if any. What I heard from you is subjective as it does not play well with the release being discussed.
You’re looking in two different places, and we do consider each separately. The tracklist – as seen on the cover (first two images) and supported by the CD – is what determines what artist credit we have. In this case, all of the tracks are given with only their name, and so we fall back on using the release artist from those same images. What you seem to be concerned about is the “Credits” list of everyone who was involved, and for us that has no bearing on who we put in the “Artist” field because most people, when looking at a disc in their hand, aren’t going to pull out the booklet and read through the small text to find every single person who played a role before telling their friend “Yeah, this is…” What that list does affect are the relationships we add on top of the recording afterward. I’m not seeing any inconsistency in this release either, but if there is something we’re missing that we haven’t covered in our explanations, do let us know.
I clearly understand what you are saying. Simply put, artist credits are to be taken exclusively from the tracklist. That’s what I wanted to hear instead of flawed reasoning and “otherwise, Britney Spears” arguments. Thanks.
I brought up the Credits list only because I was under impression (from what I was reading in this discussion) that it is important to determine if an artist’s contribution is significant enough to be credited as featured artist.
Ah, that makes sense. Sorry for the misunderstanding! But, no, our criterion for featured artists is the same as for the rest of artist credits: do they show up in the tracklist. Because of the standard formatting, that will normally be attached to the title on the art (“Title feat. Secondary” by “Primary”) and because of the workings of the database we – now – move that to the artist credit (“Title” by “Primary feat. Secondary”), but otherwise we still just take whatever the cover says. Like you said before, trying to determine “significant contribution” in any other way drops into subjectivity, while if someone official took liberties in the art, we should still be able to provide the proper data through the relationships and/or annotations.
A bit late to the party, but just wanted to share some enthusiasm… MB stores both!
All the shaky graphic design decisions and differences across releases, and then the work stores all the detail you could possibly want. If you want expert consistency, tag or display info on your app/tool/website using work relationships exactly how you want it 