Question related to Unity Samples (Hello Cube), regarding - System Update Order

I have a small question to one of the ECS Samples on Github from Unity;

I have noticed in the documentation for Entities 0.17.0-preview.41 "System Update Order" ,
that the default system group: "InitializationSystemGroup ", has the pre-defined member:
"BeginInitializationEntityCommandBufferSystem".

But in one of the examples for spawning
("HelloCube/5. SpawnFromEntity/SpawnerSystem_FromEntity.cs /"), I see that the system for spawning
is set to be a member of another group: "SimulationSystemGroup" (Which also are the default group according to documentation).

What is the reason for that? Why does it not put the spawner system in "InitializationSystemGroup"

The code snippet from Unity-Example:

Code (CSharp):

1. [UpdateInGroup(typeof(SimulationSystemGroup))]  
2. public partial class SpawnerSystem_FromEntity : SystemBase
3. {
4.     BeginInitializationEntityCommandBufferSystem m_EntityCommandBufferSystem;
5.  
6.     ....