the kind of activity focused upon in the effort to solve the story's problem
What starts, stops, and moves a story forward--this is the realm of the Story Driver. Do Actions drive Decisions or do Decisions drive Actions? The answer is yes. And like the conundrum of the chicken and the egg and which one comes first, an author dealing with narrative has to determine which came first. Cool pro tip: all your major plot point will be of the same type--either all Actions or all Decisions.
Action or Decision describes how the problem of the Story will primarily be explored. The primary concern is the kind of storytelling you want to do. If you want action to be the focus of your storytelling, choose action. If you want deliberation to be the focus of your storytelling, choose decision. It's that simple.