Hi Rachael,
by "Brain Stem constellation" we explain the psychological background of a dorsal vagal response of immobilization and mental shutdown.(Biologically, not being supplied AND being drowned in waste is life-threatening).   
In the next session we'll meet another constellation that describes a different kind of dorsal vagal response, just to mention that this is not the only one.
And yes, we may have several Brain Stem-related conflicts and still not going into dorsal vagal, this may be when these conflicts lack some UDIN feature...
Great examples, thank you!