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Explanation: I'm fairly sure this is referring to the use of a 2m rule [= straight-edge] to measure local surface variations.
I'm not TOO confident of the English terminology, but I have certainly come across this expressed as "5 mm in 2 m" --- in essence, it means if you go 2 m in any direction from point A, the height will not vary by more than 5 mm
I'm sure Bourth could give you a more authoritative answer!
Tony M France Local time: 19:19 Native speaker of: English PRO pts in pair: 14074