I can see that there are constants in the SceneTree class:
GROUP_CALL_DEFAULT = 0
GROUP_CALL_REVERSE = 1
GROUP_CALL_REALTIME = 2
GROUP_CALL_UNIQUE = 4
But the default value fails in my usage of the call_group of my project. It works with the 'realtime' value.
I would like to know what's the difference between each flag.