A World owns both an EntityManager and a set of ComponentSystems. You can create as many World objects as you like. Commonly you would create a simulation World and rendering or presentation World.
By default we create a single World when entering Play Mode and populate it with all available ComponentSystem objects in the project, but you can disable the default World creation and replace it with your own code via a global define.
Note: We are currently working on multiplayer demos, that will show how to work in a setup with separate simulation & presentation
Worldobjects. This is a work in progress, so right now we have no clear guidelines and are likely missing features in ECS to enable it.