A priority is something you grab a hold of. It’s a choice. It’s a decision.
On the other hand, an assignment is something given to you. A choice made for you.
With priorities, you get to assign the proper level of importance. With assignments, someone else already determined this for you.
Both priorities and assignments are necessary. In fact, the best assignments often become priorities.
But you have to choose which you’ll do today. And, yes, it’s always your choice.