Prep time only includes preparation steps listed in the recipe, never time to prepare or measure the ingredients. So a recipe that calls for "5 onions, finely chopped" won't count the peeling & chopping time. But when it says "sautee onions for 5 minutes" that's where the "5 minutes of prep" comes from. "Cooking time" is usually the unattended cooking time, where something is just boiling in a pot or baking in an oven.