Understanding rasterization and the painter's algorithm might help.

One way of rendering vector graphics (graphics defined by polygons, instead of pixels) to pixels is to rasterize the polygons while running the painter's algorithm.

The painter's algorithm is a bottom-up process where you first put down the background, then draw on top of that background with each layer of color until you reach the top layer.

When you deposit a layer, you pay attention to its coverage (usually stored in an extra channel, the alpha channel), and use it to mix the added color with what is already there.

If your new layer covers a pixel by 50%, and it is blue, you average the current color of that pixel with blue and draw that there instead.

Things get a bit more complex if you are creating an image with transparency, but not fundamentally.

Rasterization is the process of turning a polygon into pixels. Here, we work out how much the polygon covers a given pixel using some algebra, then calculate a coverage amount.

If you have two edges of a polygon that are coincident -- exactly on top of each other -- but both half-cover a given pixel, what happens is a problem.

Suppose the bottom polygon is red and the top blue and the background is white.

First we paint the red. This mixes with the white, leading to 50% white 50% red.

We then paint the blue. This mixes with the 50% white 50% red and we get 25% white 25% red 50% blue. The same thing happens if red and blue meet in the middle, or if blue covers red completely.

But "in reality" the blue polygon completely covered the red one, so why are we seeing it? Because the algorithm forgets sub-pixel positioning details.

So long as there is 100% coverage of one polygon, this isn't a problem.

Now, this problem is not fundamental. You can do polygon rendering with a ray-tracing like approach (where you over-render by a factor of N^2 at points), or even a pure-vector like approach (where you subtract blocking shapes from the geometry of the shapes under them, cutting them out). In neither case do "hidden" colors leak through to the output image.

The painter's algorithm isn't the only case where "hidden" geometry can leak through. If you are printing with opaque media, sometimes the color layers are not perfectly aligned. So under-layers leak through when the top layer should be covering it completely.

As you don't know how your vector image will be output, notches like that let you make images that are more robust against imperfect printing/display techniques.