public JobHandle Dependency { get; set; }

Before SystemBase.OnUpdate or ISystem.OnUpdate, the Dependency property represents the combined job handles of any job that writes to the same components that the current system reads -- or reads the same components that the current system writes to.

The JobHandle objects of any jobs scheduled with explicit dependencies are not combined with the system’s Dependency property. You must set the Dependency property manually to make sure that later systems receive the correct job dependencies.

Additional Resources: SystemBase.Dependency