I didn’t know about this system when I first moved to Amsterdam, and it was just luck that I met the window washer in the street one day and asked him how it works. He cleans my windows on the street side of the building once a month, and I pay him 10 euros.
He’s not a young man, but he’s quick up and down the ladder, and obviously has good balance as he stretches out to wipe the windows. He works in almost all weathers, so even on a cold cold day he’ll be out there, with a cigar clamped between his lips as he whips and down his ladders.
Apparently the tax department estimates his income based on the area he covers and the number of windows in that area. That’s right, the tax department knows how many windows are in my street.