Worm drive hold torque vs. stall torque
Worm drives are unusual gearboxes. 

Because of their capacity for high ratio, they can have very low efficiency and still be useful. It's quite difficult to make an efficient worm drive gearbox, you need a low ratio, and usually a multi-start thread on the worm, and often special low friction materials.

If a worm drive has an efficiency of less than 50%, it's impossible to 'back drive' it, that is turn the output shaft and spin the worm. In this case, the 'hold torque', if that's defined as the maximum torque that will be held by the output shaft with the input motor unpowered, would be infinite, or the breaking strength of the gearbox.

As that motor has fairly high power, and specifies a hold torque less than its max output torque (obviously much less than gearbox breakage level), it looks like the efficiency is > 50%, and it can be back driven.

The hold torque is now governed by how much torque it takes to turn the unpowered motor. This will be governed by friction and motor cogging. The first will be poorly defined, depending on lubrication state and bearing smoothness, though the second will be more predictable. 

The manufacturer here is giving a torque figure that he expects will always be unable to spin the motor. Given the uncertainties in motor friction, I would not be surprised if you typically need to apply rated torque or even overload torque to get the output shaft to move.