What are the differences between EntityCommandBufferSystem classes?

BeginInitializationEntityCommandBufferSystem
BeginPresentationEntityCommandBufferSystem
BeginSimulationEntityCommandBufferSystem
EndInitializationEntityCommandBufferSystem
EndSimulationEntityCommandBufferSystem

How do I know which system to use to get a EntityCommandBuffer? Also there is PostUpdateCommands to use on ComponentSystems which seems to get destroyed after OnUpdate(). Do I ever need those systems when I can use PostUpdateCommands.

As a second question, how do I know if I need to use EntityCommandBuffer or EntityCommandBuffer.Concurrent?

 

https://docs.unity3d.com/Packages/c...ystem_update_order.html#default-system-groups