The term RTI stands for Ramp Travel Index. Ramp Travel index is a measurement of a vehicles suspension, frame and tire compression and the resulting flexibility of all components of the vehicle to achieve a score. This standardized test uses a ramp to find the extreme flex point of the vehicle.
The RTI test is a good indicator of how well a vehicle will do in many situations offroad while traveling over an obstacle. Ideally a vehicle will do better if it is able to keep all wheels on the ground for maximum traction. If a vehicle's suspension is too stiff (less flex), it generally will lift a wheel or possibly two wheels while attempting to get over an object resulting in a loss of contact and traction at those lifted wheels. Loss of traction is especially apparent with vehicles equipped with open differentials. In situations requiring a lot of flex locking differentials and some limited slip differentials will help maintain momentum with the opposite wheel of the axle (front or rear), however good flexibility results in more wheels on the ground, better traction and better stability of the vehicle as the suspension conforms to the terrain.