If you query a release by ID and include both discs and recordings the result contains two mediums, one with a filled disc-list and empty track-list, and one with an empty disc-list but a filled track-list.

Example:
http://test.musicbrainz.org/ws/2/release/23d752fa-d27c-33bf-ab33-aff5fb47d507?inc=discs+recordings

This might affect other combinations, too, but I have not tested that.