An entity can be assigned to an AppStack. Entities can be Users, Computers, Groups, or OU (Organizational Units). The Assignments page for VOLUMES (2.X) lists entities only which are assigned to AppStacks.
By using the Assignments tab, you can view the list of entities assigned to all the AppStacks.
For more information about assigning and attaching AppStacks to an entity, see Assigning and Attaching AppStacks.