Front, Back and Spine types are not meant to be used for front/back of CD, front/back of inner things like liner or inner jewel cases, front/back of everything and anything.
Front, Back and Spine types are for the outer packaging, IMO.
But recently I am starting to see some exceptions to this custom.
The guidelines should precise this (they don’t).
What do you think, @reosarevok?
What would you use for the front/back of individual CDs within a set, @jesus2099?
Just Medium for the CD label and Medium+Matrix for the other side, if we can read the CD matrix.
Or if you mean the inner jewel cases, then Booklet can be used and Other, and we can say what in the comment.
But having several Front and several Back, then I don’t understand the packaging at all, I think there is a mistake.
I remember reading about using them only for the actual front and back of packaging, when CAA began…
But as it’s not in the wiki, then maybe it was either in the old firm or even rather in the old making list.
Cover Art / Types - MusicBrainz does specify that front is “The album cover, this is the front of the packaging of an audio recording” and back is “The back of the package of an audio recording, this will often contain the track listing, barcode and copyright information”.
For front and back of a CD, @jesus2099’s suggestions make sense to me (I assume there’s something interesting on the back of the CD since that’s the only reason I could see to scan it)
So the comment will say back cover of CD 1 but the type won’t be back cover? Seems strange to me.
This packaging is basically 4 jewel cases wrapped up in a paper box. I think they did it so to be able to sell the CDs individually as well.
Oh, no, sorry, I missed the specifics. I wouldn’t use it for things like the medium, but for back and front of individual jewel cases it seems fine. Just make sure the front of the whole packaging is the first ordered Front image.
If we should distinguish the real Front by putting it first, then what is the best way to distinguish the real Back?
We should put it first? Last?
Will this implicit distinction be enough to let people understand the actual packaging setup?
For me, putting additional Front and Back on some things inside the packaging is a little bit confusing.
But now that I know it’s acceptable, I will keep that in mind when I review cover art edits.
I would put it last, although that’s not as important since it won’t affect the sidebar image 
You can require a comment on the additional front/back.
But the order of the Backs affects the special URL with /back at the end:
So, apparently, real Back should come before all other Backs, like real Front should come before all other Fronts.
It used to be you showed the box front\back\side\top\bottom as a little set first. Then the various CDs as separate little sets.
I’ve always followed the way @tigerman325 did these: https://musicbrainz.org/release/d75185ca-9f59-47a7-b360-a5303d9ca1aa/cover-art
That way the front (2), back (2) generally stick together in useable batches in a download.
When something is as huge as this: https://musicbrainz.org/release/f881400f-1e7b-46d7-8374-adf4ffd1f4ea/cover-art it feels easier to keep track of when they are together.
I did mean the inner jewel cases; sorry for the confusion. To me, it would be strange to call the front and back covers of a CD packaged in a jewel case one thing, but to call them something different if that jewel case was also packaged in an outer box. I think it’s a good practice to use comments to identify which “front” is which.
I do generally follow the order of front+back (+spine etc. if adding) of the outer packaging first, followed by individual media. Maybe this should be added to guidelines so the URLs @jesus2099 mentioned will work consistently.
Honestly I had entirely forgotten we have a /back URL! So yes, we probably should be more specific about ordering then…