EventBus usage is not clear

[Shadowblitz16]

The event bus documentation is not very clear.
Does it support multipule callback?
Does it support multipule callback per class?
What is e?
What does send take?

            var e = Load;
            EventBus.Send(ref e); // error
        });

        World.Query(windowStateDesctiption, (Entity entity, ref WindowInfo info, ref WindowState state) =>
        {
            
        });
    }

    [Event(order: 0)]
    private void Load(ref LoadEvent load)
    {
        
    }
    
    public readonly record struct LoadEvent   (float Delta);