That's a stack, and on the left is the pick, the image that I actually want to use out of all the candidates. When the stack is closed, only the pick shows:

But there is another kind of pick supported by Aperture, an Album pick. Album picks are very handy because they let you have a different pick for each album (or gallery, etc.) that the image appears in.

If I open the stack again, duplicate the K twice (option V) and put those two K versions together in another stack, then put that stack in an Album by selecting the images and selecting New From Selection -> Album, I get this:

Working in the album and duplicating one of the versions I get this:

There is a check mark on the third image. That check mark shows the Album Pick. You can set and clear the Album pick from the Stacks menu (Stacks > Set Album Pick) or use command shift backslash. An Album pick allows you to have a different pick for each album that the stack appears in. This is useful because the context of each album may cause you to favor a different image for each one.

To illustrate this, I created a stack containing three versions in the Project called Stack. I made some adjustments to these three versions so that they are visually distinct; blue, yellow, and rotated:

Here is the same stack in an Album (note the check marked image):

And in another Album (note the different check marked image). I did this by selecting each Album making different images the album pick for the Albums:

If I close these stacks I get different results in each case. I get a stack in the Project with the blue image showing as the pick, as expected:

In the first Album I get the rotated image as the pick:

And in another Album I get the yellow image as the pick:

This shows the strength of stacks: they are a tool designed to defer choosing among interchangeable images to the point of use.