The recent shooting incident in a mall in Portland, Oregon is a reminder that first aid is necessary when you least expect it. In a case like this, responders will be burdened with deciding who gets aid first and in some cases, who lives and who dies.
It's called a multi-casualty incident (MCI) and it means there are more patients, especially critical patients, than there are resources to easily handle them. In that case, there will have to be some sort of rationing. To ration resources effectively, responders have to decide which patients will get the most good from treatment and focus on those patients first. This method of sorting is known as triage.
Once we've decided which patients to treat first, paramedics are still not done. We must decide which injuries to treat right away and which ones to ignore until later. Prioritization is the most important organizational skill a paramedic has. It's a hallmark of good EMS leadership.