ISO 3310 Clarification Help - Aperture sizes for sieves used for particle sorting

#1
Hello,

The ISO 3310 standard defines aperture sizes for sieves used for particle sorting. I'm looking at one of the tables that defined the tolerances of the apertures and I'm a little confused by what it says. It provides the following information for each nominal aperture size.

"For any aperture size (+X)" with X values stated.
"For average aperture size (±Y)" with Y values stated.
"Maximum standard deviation (sigma_0)" with sigma_0 values stated.

What I don't understand is how I should read the first two columns. For a given aperture size, would I automatically add the X value, which is then toleranced by Y? Any insight into this would be much appreciated!
 

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