I'll take a guess at it before someone who actually knows the whole answer replies.
There must be some standarized test to see how much weight a stepper motor can lift and/or hold in position. Maby there is a standard shaft diameter, maby the existing shaft on the stepper motor, or maby 1 cm, or maby 1 inch sized shaft or wheel. Maby for example a 30 oz rated stepper motor can hold a 30 ounce weight and as the motor steps through its rotaton movement. Thats my take, I hope its close. Also, external gears to the stepper motor can alter the speed and torque of the power available for use.
From Wikipedia, but it does not say how the torque rating or weight test is made: http://en.wikipedia.org/wiki/Stepper_motor
Theory: A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and stator) increased, taking care that they have no common denominator. Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design, having both permanent magnets and soft iron cores.
To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step. Winding inductance and reverse EMF generated by a moving rotor tend to resist changes in drive current, so that as the motor speeds up, less and less time is spent at full current -- thus reducing motor torque. As speeds further increase, the current will not reach the rated value, and eventually the motor will cease to produce torque.
Stepper motor ratings and specifications
Stepper motors nameplates typically give only the winding current and occasionally the voltage and winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the motor rated voltage.
A stepper's low speed torque will vary directly with current. How quickly the torque falls off at faster speeds depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.
Steppers should be sized according to published torque curve, which is specified by the manufacturer at particular drive voltages and/or using their own drive circuitry. It is not guaranteed that you will achieve the same performance given different drive circuitry, so the pair should be chosen with great care.