The trajectory of the reader. The defining feature of hypermedia is for a branshing and interconnecting reading experience. Non-linearity and engagment is the essence of hypermedia.
The relationship between the author and the readers. Readers can add to the materiel and make there own connections. Indeed they can use the media as a space that facilitates and shapes engagement with not only the intention of the author but with each-other. In so doing, the generate the text of the medium its self. In the limit, each reader is as much an author. Both are users, and users are both. Thus hypermedia becomes social-media
The relationship between adjacent medium. Since media can interconnect with one-another, if the do so at sufficient density, they form a larger hypermedia. Thus hypermedia becomes transmedia.