The Tier-0 Uplink interface is used to connect the Edge node to the external network. The Downlink Interface for the Tier-0 DR is used to connect the Edge node to the Tier-0 gateway.
The Inter-Tier interface on the Tier-0 gateway is only used in a Dual Tier topology. The Tier-1 SR Router Port and the Downlink Interface for the Tier-1 DR are only used in a Tier-1 topology.