Any effort requires supplies, often called essentials. The effort to achieve the Goal also requires these essential Prerequisites. You can't make progress without meeting the Prerequisites. Only by gathering the needed materials can you try to meet a story's Requirements. Prerequisites might be a certain kind of transportation, a specific amount of money, a grade point average, or the approval of a bureaucrat. As long as the item in question is essential to mounting the effort to achieve the Goal, it is a Prerequisite. Prerequisites themselves do not bring the Goal any closer, which is why they are not Requirements. All they do is define the raw materials or foundations that must be in place before the quest for the Goal can advance.