Use ～と決まっている when stating something as fixed by rule, precedent, or the nature of things — company policy, standing schedules, or the kind of outcome that always follows. In business writing it carries an impersonal, non-negotiable tone: the speaker isn't deciding, merely reporting a settled fact. The inevitability sense ("it's always the case that…") is close to 必ず～ and often appears with a hint of resignation.