Collaboration normally takes place in some kind of meeting, which can differ in interaction protocols, group size, formality, etc. Each participant of the collaboration can have one or more roles depending on the sort of meeting. A role is a set of rights and obligations (Ellis et al., 1991). We distinguish the following roles: chair, listener, talker and interactor. The chair sets up the session, a listener is a passive participant, a talker is actively explaining his arguments and, finally, an interactor is able to interact with shared resources. The rights and obligations of the different roles are determined by the interaction protocol. The possibility to switch roles dynamically is important, since a listener can change into a talker from one moment to the other.
Collaborative visualization in DIVA is a virtual meeting, where the participants are at different places and their desktops are connected by a network. We will call the event of such a virtual meeting a session. Session management should support several kinds of sessions and thus be able to handle changing numbers of participants, their roles and interaction protocols.
The notion of subgroups makes it feasible to split the total group of participants in (non disjoint) groups. These subgroups can communicate separately or perform subtasks. Subgroups can come into existence dynamically.