construction and used components will define maximum stress arm can endure. they are selected based on requirements for specific model. this can get you pretty good info what may be possible with specific design. normally if you gradually increase load you will reach point where system is not able to react as expected. then you can do stress tests to confirm that robot system is able to operate in certain condition over long time without overheating, premature wear etc. The actual rating you list in specs is data you come up with - it is compromise between performance and reasonable service life.
Thanks a lot, wise words! Could you or other pals provide appropriate sources from which I could read more on the subject? On topics like robot building, load testing, and stress testing. And other answers are more than welcome. It would also be useful to look at this in more detail.