Basic Emerson Terminology
The tutorials and documentations throw around two terms that are unusual, and should be defined. These are entity and presence. Here's a quick description of the relation between entities and presences.
An entity has an associated Emerson script. It has zero, one, or many presences through which it connects to a virtual world. Each presence has a physical manifestation in the virtual world, and has properties such as velocity, mesh, bounding volume, etc. that are managed by the virtual world.
As a concrete and visual example of entities and presences, consider the following figure
In it, Entity A has two presences associated with the world, both fish. Each of these fish can move independently of the other in the virtual world. In contrast, all the other entities only connect to this virtual world through one presence. (In case you're wondering those presences are seaweed, a starfish, and another fish for Entities B, C, and D, respectively.)