Great question, and I will have to dig for more info. But in short, higher elevation IS a concern with any vacuum lifting or holding device. The higher you go, the less atmospheric pressure you have. Considering that these use vacuum (negative pressure) as the primary source for holding force, it can and does reduce potential capacity.
The key here is the relative rating of the device, with respect to it’s ultimate holding force at any given elevation. If you give enough Design Factor to something, you can rate it and not worry about anything you can drive too. But, the DF is not universal and the relative rating is actually dependent on a range of factors to include the CoeF between the pad and material, as well as things like bending moment and dynamic loading.
We have specific ratings for all of our lifting products with this at the core, so it’s very valid to consider. I have not dealt with the specific considerations of a rack since we don’t sell those, so I’d be guessing as to their relative ratings. As above, following MFGR specs is likely best. If you have questions beyond that, I’d ask them directly.
ETA:
For reference, here is one example of a “regular” product spec for one of our hand cups:
We also offer a “high elevation” version of this with a different vacuum pump installed, and a reduced 100lbs [45 kg] capacity up to 12,000 ft [3,658m]. So it’s possible to handle higher elevation with some pump changes and likely a reduced capacity.