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HelloI don't know about IEC, but for the ANSI standards, there's no tolerance. The maximum is the maximum. From a practical viewpoint, you have to take into account the accuracy of your short circuit calculation. There are generally impedances that are neglected in most calculations, such as impedance in breakers, terminations, etc that can tend to reduce the actual fault current slightly. You might be able to sharpen your pencil and come up with a slightly lower worst-case calculation.
But if you are the responsible engineer, then you probably can't just say that it's so close, I'm sure it is fine.
Yes, main question is if there are any tolerance by IEC or IEEE.From the manufacturer, it depends on whether they are testing to meet the rating (e.g. Ik3" = 40 kA) or if they are testing to see when it actually fails (Ik3" > 40 kA).
As a user - design around it passing ONLY the maximum value(s), but no more (i.e. no "higher tolerance"). That is the conservative and safe approach - and, as an engineer, one of your prime responsibilities is the SAFETY of the system with regard to both equipment and personnel.