Despite all the Agile approaches which become hard to follow in a multi-project context all for themselves following Agile, depending on culture but also real life project situation, I observe over years that if there are racing conditions in context of understaffing or unexpected troubles, be it due to internal or external dependencies, the result is, plannability and reliability of delivery declines.
Sometimes, when it is about prioritization, it's some "priority one" task. Then there are many "priority one" tasks.
Therefore my question is:
How to properly* disambiguate how much resources "priority one" shall consume? 80%? 100%? Moreover, does "priority one" mean, "we should stop all other tasks"?
How to properly* disambiguate how to split time between multiple highest priorities?
* Properly should mean here not "what is your opinion" but "do you know a source where this problem is described and assessed" either "have you experienced and addressed this problem"?