The Tiers represent the expected efficiency of the graduates during the event according to their studies level. It's like a filter for the student's order from best to worst in a determined subject. Tier 1 shows the most suitable ones for the tasks given in the Humanities event, for example, and the rest of the Tiers show the less suitable ones for that same event. You still can choose graduates from any tier, but their performance can be predicted according to the Tier they are shown, so we recommend you to take this classification into account. You cannot improve these already graduated students, but you can prepare the future ones to be better at one specific stat in your University.