You can use Multiplayer Play Mode (MPPM) to test multiplayer functionality within the Unity Editor. You can simulate up to 4 players (the Main Editor and 3 Virtual Players) simultaneously on the same development device while using the same source assets on disk. MPPM can help you create multiplayer development workflows that reduce project build times, run it locally, and test the server-client relationship.
The following have specific meaning in relation to MPPM:
Main Editor: The original instance of the project in the Unity Editor. This is the only instance with full authoring capabilities. Virtual Players: Simulated players created with MPPM. These players open in a separate window with limited authoring capabilities when you enter Play mode. Players: All player instances, including the Main Editor Player and all Virtual Players.
The Unity Editor and Virtual Players require a lot of system resources, so you shouldn't use Multiplayer Play Mode (MPPM) at scale. MPPM is designed for small-scale, local testing environments that can only support up to 4 total players (the Main Editor and 3 Virtual Players).
Virtual Players have restricted authoring capabilities because they're intended as a vehicle to open multiple project runtimes, not provide a multi-editor authoring workflow. You should use the Main Editor to make changes and the Virtual Players to test multiplayer functionality.
You can't access any Main Editor functionality from Virtual Players.
To reduce the demand on system resources caused by each Virtual Player instance, MPPM shares specific resources, such as the artifact database and imports between the Main Editor and each Virtual Player.