This is a specification of the quality of the printing. Look closely at a black and white newspaper photograph (a 'half-tone') and you'll see thousands of little dots. If the picture meets a specified quality, the size of the dot - near enough, within the tolerance of the spec. - is the same as 'the target print area'. Lots of big dots trick the human eye into seeing that part of the picture as black.
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.
b





