In your case, 85% of the dots must be at or very close to the target print area. 10% of them can be between 75% and 125% of the target area. And 5% of them can have between 50% and 150% of the target print area. (I'm guessing here, but I think there's a "±" missing). And this quality standard must be maintained throughout a square 50mm X 50mm (that is, a big picture doesn't meet the spec. if most of it is perfect but a square that small is substandard).
I don't know how this would translate to glass printing, but presumably the principles are the same.
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