Seniority is used to specify the order in which payouts are given in an exit event. This gets used in calculating Breakpoints for the Waterfall and other scenarios. These are listed in ascending order, meaning that the most senior (the security to first receive a payout) is the one in the bottom Tranche.
To access seniority, just go to Securities Classes > Seniority from the side menu. Once there, you can click and drag securities into the proper order.