One of the most powerful innovations that Apple introduced with Aperture was the stack. While it’s very easy to think of a stack as a folder - and stacks can function very well as a container for related images - this isn’t really what they were designed for. Ultimately, the point of a stack is to provide access to the one image out of a group that you really like. That image, the pick, is shown on the front of the stack. The rest of the stack can contain alternate images that are there if you need them, but the idea is that you probably won’t ultimately ever utilitize these images - the stack simply provides a quick way to access them should you need to swap one out with the current Pick.
With this philosophy in mind, Aperture really intends for you to think of a stack as a single image - the Pick. As such, it alows you to handle a stack just as you would a normal, unstacked, single image. You can drop it in an album, a book, or a web page - or start editing - and only that one image will be altered. This is a fundamental understanding that Adobe has missed with Lightroom, as evidenced by the fact that stacked images don’t present themselves as such in many parts of the program.
Unfortunately, it’s also an understanding that slipped by Apple when they created Aperture’s Export Masters command. If you select a closed stack with the intent of exporting the master of the Pick image, Aperture will export everything in the stack! if you’re working with a stack containing a lot of high-res images, this export can take a fair amount of time and disk space, and leave you with a bunch of images laying around that you need to sort through and delete.
Luckily, the work-around is fairly simple: just open the stack, select only the image(s) that you want to export, and choose the Export commands as normal.