If you have a cd sold as a box (like the complete work of an artist), do you create one release for the whole box or, as I see for some discs, add them as separately. Is there a guideline for this?
I first thought it should be the same as for instance when there’s multiple CDs in a jewel case but one problem is that in the case of a box with separate packaging for each cd, you may have a different cover for each disk.
The guide is to have one release with everything and a medium for each cd.
See the following thread for discussion on what to do with this sort of thing.
2-in-1 release groups and other boxsets (STYLE-330, STYLE-331, STYLE-335):
See http://musicbrainz.org/series/b43b4a74-52d5-4d79-98d9-be3188176a68 for examples of this sort of thing.
The reason you sometimes see this is because the individual discs have also been released on their own, and not just as part of the box set that also contains them. Unless you have the CDs released individually, you should just add the entirety of the box set, as @dns_server said.
What I have encountered is this one
where some of the discs in the box have been added a separate releases with an annotation that says something like
This belongs to the box “…”. This CD is not sold separately
@dns_server thanks for the clarification. But then there is no way to have a separate album art for each medium, right?
Currently it is not possible to set cover art for each medium see:
Cover art in musicbrainz has a type flag and an order so my suggestion would be to just add multiple front covers.
It would then be possible to add a feature to picard that when there are multiple front covers it must be a per medium cover and pick them but this feature does not exist.
Ok, I added the box as one big release with multiple medium, what should I do (if anything) with the existing releases for separate discs?
In this case, we do like PORTAL 2, for instance, reusing same recordings whenever appropriate.
Couldn’t see a response to this, so wanted to mention that releases like this are simply incorrect, and should eventually be fixed.
AFAIK most are from before MB had support for multiple mediums in a release?
I found out that MB allowed me to directly re-use the existing medium, which was really nice 
Fixed how?
The “stand-alone” Releases should be merged into their box set Release:
https://musicbrainz.org/doc/How_to_Merge_Releases