SubSystem

This refers to a technical concept, not as a generic "container subsystem" or a "interactions subsystem"

A subsystem is a class that manages something, it should work on its own, the only exception is when it needs information about other subsystems, but it is preferable that you do that by using events, event buses, or network messages.

A subsystem can be networked or not, for that you can inherit your class from SubSystem or NetworkSubSystem.

Here's a snipped of how you can declare a subsystem that creates an explosion somewhere.

public sealed class ExplosionSubSystem : NetworkSubSystem 
{
    public void CreateExplosionAt(Vector3 position, float size) 
    {
        // boom.
    }
}

It is also important that you know how all the subsystems work with the SubSystems class

Always add the postfix "SubSystem" in the name of the class for your subsystems.