When you draft a business plan, you have to make many different types of assumptions. Some of these are so basic that they remain, appropriately, unstated. Realistically, there is no point in worrying about cataclysmic or other events that can render all your planning moot. You have to start with the assumption that it's going to be "life as usual." For example, retailers assume that consumers will continue to make most of their purchases during the holiday season.
Beyond that, there are several broad types of assumptions that you're going to have to make. These assumptions are what support and quantify the projections that you'll make in the plan.