In draft-gregorio-atompub-multipart-04.txt chapter 3, third paragraph it says :
A multipart/related POST to a Media Collection MUST be a valid multipart/related representation as defined by [RFC2387] and MUST contain two body parts. One, referred to as the Entry Part, MUST be an Atom Entry with a media type of 'application/atom+xml' or 'application/atom+xml;type=entry'. The other, referred to as the Media Part, MUST be of a media type acceptable to the collection. The object root MUST be the Entry Part. The Entry Part's atom: content element MUST have a 'src' attribute whose value is the URI of the related media in the Media Part. The 'src' attribute value MUST be a 'cid:' URI as defined by [RFC2392]. The Content-Type: header of the POST request MUST specify "application/atom+xml;type=entry" or "application/atom+xml".
The question is, what does "MUST containt two body parts" mean? Does it mean that it must contain at least one Media Part, or that it must contain one, and only one, Media Part?
It means it must contain one Media Part and one Entry Part making two body parts in total.