According to Oliver's answer in http://lists.musicbrainz.org/pipermail/musicbrainz-devel/2010-March/003790.html , only Artists, Labels, Recordings, Release Groups, Releases, Works and URLs will be able to have ARs.

Medium is not a part of the "core entities", as described in that reply. This leads to 2 problems.

First, for ARs which currently are not "fuzzy", such as described in http://bugs.musicbrainz.org/ticket/5501 , in NGS, they now will become fuzzy. That AR is not fuzzy at the current release level, which will become the Medium level in NGS. By not having Medium AR support, and instead moving all current release ARs to NGS Release ARs, there now is no way to specify why did what on a specific medium (or mediums) within a NGS Release; the AR can only be resolved to either a track (not applicable in the case of this AR) or a NGS Release (which assumes the AR applies to all mediums within the Release).

Second, for ARs which are fuzzy at the release level, the same type of lack of resolution problem happens. For example, take a 10 CD classical box set. Each disc lists the orchestra and conductor, but no per-track detail is provided on the liner. Currently, this is represented in a fuzzy manner, by applying the ARs to the particular release that represents that particular disc. In NGS, with these ARs now moving to the NGS Release level, we can no longer identify the conductor and orchestra to that degree of fuzziness. Instead of specifying those ARs at at least the Medium level (some fuzziness), we now will be forced to specify those ARs at an even higher level of fuzziness, up at the Release level. So for this box set, I could easily now get a Release listing 10 conductor and 10 orchestra ARs for the Release, with no way to see which of each set of 10 applies to any given Medium.

This would be simply solved by adding the Medium entity as one which can be involved in an AR.