Are these three separate layers?#
No. They are responsibilities and do not have to be separate layers or separate people. A single manager can hold all three in a small organisation; what matters is that each one is held by someone, because they pull in different directions and the one that is unowned is the one that quietly loses.
Why must decision rights be explicit?#
Because the three conflict routinely. Mission leadership wants the outcome now; people leadership wants someone to learn by doing it slowly; system management wants the rule that applies to every mission. Decision rights must say who decides when they conflict, written down before the conflict arrives.
System management is the newest of the three, and the one with the least existing practice. Allocating attention across missions is a management job that barely existed when machine capacity was the scarce resource.