It could be useful to have an “Excerpted From” RG-RG relationship. Sort of the inverse of “Included In”, but “Included In” implies that the included RG is in more or less the same form as its standalone version.
I’m thinking of cases where there’s a multi-disc set, and then a single-disc selection of tracks from that set. For example, Early Ellington and The Best of Early Ellington. All the tracks on the “Best of” release also appear on the larger set, but interspersed among the other tracks.
I guess it’s kinda-sorta the same discussion as Highlights releases: same or different RG? - which was not super conclusive. I do still feel these should be their own RGs and in that case they should have their own relationship to match them as you suggest, but not everyone seems to agree 
my thought is if there’s a relationship that can be added between the two, I see no issue with seperate release groups, in the case of the above, highlights, and album samplers
Separate RGs is the way to go, I think. When the “Best of” release has only a third or a quarter of the tracks of the full set, it doesn’t make sense to consider them the same.
Linking to the ticket discussed in the thread @reosarevok cited above: [STYLE-1241] New release-release relationship: Highlights - MetaBrainz JIRA
I took an executive decision and added this:
https://musicbrainz.org/relationship/6d3242b9-0eb5-45c7-bf07-303aaff46d0f
If there’s grey areas where the community considers an excerpts release belongs in the same RG, then obviously this won’t be used but the two releases will be linked by being in the same RG, but there’s more than enough examples of separate RGs to justify adding this.
Should the wording of Relationship type / Single from - MusicBrainz be clarified if this other relationship’s guidelines explicitly say to use it for “promoting” rather than “taken from”? “promo” doesn’t even appear there.
Furthermore, “Compilations and soundtracks should not be marked as “taken from another release group”, even if they contain singles from them.” is misleading advice, because many labels put out mini soundtracks prior to the release of the full soundtrack that are intended to promote the future bigger release. (That differs from a selection of a soundtrack released after the full album which serves as an excerpt rather than a single/EP that promotes an upcoming release.)
Can a release be an excerpt of multiple releases? For example, is https://beta.musicbrainz.org/release/2972ff4d-d017-4cb6-b7ea-6947dbb9ab18 an excerpt of 4 releases, or zero?
Well I would say: just try!
I think it should be possible, indeed.
It also means zero relationships, doesn’t it?
There would still be relationships at the recording level, of course.
I think “excerpt” implies a one-to-one relationship. Beyond that, it’s just a compilation. Taking @yindesu’s example to an extreme, you could say a “Best of Artist X” is excerpted from all of Artist X’s releases, and that just seems silly.
I would expect them to be the same recordings in most cases. If they weren’t, they might be linked to the same MB Work, but most likely would not be linked to the other MB Recording.
I don’t fully understand the need next to the RG-RG includes that we already have. 
All relationships can already be read in two directions.
PS. I will have to add this new Excerpt from relationship in COLLECTION HIGHLIGHTER, like I did with includes.
My interpretation:
Smaller releases that are collected in their entirety into a larger release = “included in”
Larger release that is trimmed down to something smaller = “excerpted from”
You’re right, of course. I was actually thinking in terms of shared recordings so I don’t know why I wrote about relationships. 
Here is an example of includes:
I’m pretty sure that if you find for me an example of excerpt, it would have been posible to use the includes (backwards) relationship on it, already. 
Since you’ve said you don’t understand - here’s an example where the excerpted release is even named “Excerpts from…”. The excerpt is “only” 10 tracks from a 55-track release, so the releases are different enough to require separate release groups.
another example is album samplers like these
that said, I don’t know if all “samplers” should have this relationship or not, for example, label samplers like these (not familiar with the Wyndham Hill release, but the Take Time to Listen samplers are basically regular label compilations, just of new music)
although, perhaps a relationship specific to sampler releases (the former, not the latter) might be in order? not super important in my eyes, but I’d be down for it
The line in the description is just meant as a generalization (singles have historically usually been used to promote an album but I understand that’s not necessarily the case in Asia for example and nowadays it sometimes isn’t the case in Western markets either). If you feel it’s confusing I can reword it, but I just wanted some easy “this is clearly not an excerpts release, go there instead” wording here.
That’s why we have guidelines and not rules The point here is that often a compilation or soundtrack can include a song that later gets released on an album but that is not enough to link them as single/EP from - your use case seems to explicitly be an EP promoting a release, in which case common sense says use this.
I’d say this is just a compilation, not an excerpts/highlights release, although it could be linked as a highlights release if there was also a boxset with all of the full soundtracks together the compilation was released alongside.
Thanks for the example.
I would have been perfectly fine with the pre-existing relationship:
Excerpts from DISSIDIA FINAL FANTASY Original Soundtrack is included in DISSIDIA FINAL FANTASY ORIGINAL SOUNDTRACK
DISSIDIA FINAL FANTASY ORIGINAL SOUNDTRACK includes Excerpts from DISSIDIA FINAL FANTASY Original Soundtrack
The new relationship is just another way to say the same thing, IMO, so it feels kind of bogus to me.
But maybe it’s only me.
I don’t know if this will help, but: all the tracks of the “excerpt” are included in the release from which it’s excerpted, but not typically in the same order or context. An “included” release should appear in basically the same form within the included release as the standalone version.
Could be but it’s not part of the documentation and I never limited myself to this case when using this relationship.