I don't know if this is official, but AT probably stands for Automatic Truck, MT for Medium Truck, and HT for Heavy Truck.
These correspond with the 500-series, the 600-series and the 700-series.
If you have an HT-700-series tranny in your Blue Bird, then your engine puts out more than 250 HP and/or 640 Pound/Feet. In other words, you have one hecka bus!