Multiplayer Play Mode is a feature that enables multiple editor instances to be opened simultaneously on the same development device.

Multiplayer Play Mode enables you to test multiplayer functionality without leaving the Unity Editor. You can simulate up to four Players (the Main Editor Player plus three Virtual Players) simultaneously on the same development device while using the same source assets on disk. You can leverage Multiplayer Playmode to create multiplayer development workflows that reduce the time it takes to build a project, run it locally, and test the server-client relationship.