any kind of resource can be listed in a collection (http://tools.ietf.org/html/rfc5023#section-4.2), and everything that is in a collection is represented by an entry. entries follow the standardized metadata model defined by atom (http://tools.ietf.org/html/rfc4287), but atompub distinguishes two kinds of entries. if a client POSTs an "entry resource" (an atom entry following atom's metadata model), the server pretty much takes this entry resource and starts listing it in the collection as a member. if a client has POSTed a "media resource" (pretty much anything that's not an entry by itself, often something like an image media type), which has to be in one of the accepted media types of the collection, the server accepts this media resource and creates a "media link entry", which will then represent this media resource when you list the collection contents. servers might try to be smart about populating metadata fields in the entry and for example look at exif data to populate certain fields. the interesting aspect of this setup is that you POST one thing, and create two resources (http://tools.ietf.org/html/rfc5023#section-9.6), and big media files might for example get added to a CDN and get a CDN URI, whereas the entry gets some URI under the control of the atompub server. [Model Aspect] 
creating collection members