AC 3 tier accommodation denotes sleeping accommodation which has 3 berths stacked one above the other - Lower Berth, Middle Berth, Upper Berth. The berths are used for sitting during day generally and for sleeping in the night.
Chair Cars are for travel by sitting only in Cushioned chairs.
AC 2 tier accommodation denotes sleeping accommodation which has two berths stacked one above the other - Lower Berth and Upper Berth only.
Garib...
more... Rath trains have only AC 3tier accommodation and Chair Car coaches. GR does not have AC 2 Tier coaches.