Truggy was originally a buggy converted into a truck. MSR did conversion kits originally and some others as well. The concept caught on and has become one of the fastest growing classes in RC racing and now most manufacturers make specific "truggy's" now that are their own design and not a conversion. The class is also getting away from being called truggy or unlimited monster truck and more and more it's being defined as Arena Truck. Arena Truck class consists of:
Mugen (either the MSR conversion or the MBX5T)
Kyosho ST-R or the RTR version
Jammin CRT
Hot Bodies LSP
Ofna has a the Mutilator and something else
Production monster truck would be:
Losi LST / LST2
Revo
Savage
et all
The big difference and what will determine where you run is whether or not the truck has a gear reduction transmission or a center dif. As defined by ROAR or RC Pro rules the Revo while it can have a center dif, still has a gear reduction tranny so it runs in the Production MT class. If you run a BB in Prodcution MT there's a minimun weight. Also the size of the fuel tank is regulated.