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


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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