An RLI is essencially an outbound call control table that, based on settings from the originating TN, determines whether or not the call is permitted to be sent off the switch and what route (trunk group) the call will be sent out. This is a simplified description. RLIs can actually do a lot more- like chosing a secondary route if the primary route is not available or busy, altering the digits dialed from a user before sending the call out, selecting different routes based on the NCOS settings of the originating TN, etc...