Understanding Version Sets, Stacks, Picks, & Album Picks

Question

Aperture's documentation is very good in most respects (adjustments being the major exception), but I found it very difficult to put my head around the whole version set / stacks / picks / album picks thingy.
While working on some images this morning, it all became clear.
1. Version Sets vs. Stacks
A Version Set is a single Master image with two or more Versions.
Version Sets have a light gray border around the contained images, and a single dark gray border around all those images in the Version Set.
A Stack is a collection of versions that do not use the same Master.
A Stack can contain single Versions or Version Sets (and Version Sets remain grouped within a Stack: you'll see a light gray border around the Version Sets and a dark gray border around everything in the Stack).
If you Stack two or more Version Sets, and then choose the Unstack command, all the Versions become separated--even if they share the same Master (i.e., UnStacking a Stack does not result in Version Sets, as you might expect--instead, it results in all individual Versions).
You can re-Stack these separated Versions that share the same Master, and you'll get a Version Set (light gray border surrounded by dark gray border) automatically (which is good, since there is no Create Version Set command).
From all these results, it appears that a Version Set is simply a Stack where each Version shares the same Master. It is different from other Stacks in that these Versions have a common light gray border.
2. Picks vs. Album Picks
Create an Album and add two separate images (i.e., two Versions with different Masters). Call it Album1.
Duplicate the Album. Call it Album2.
In Album1, create 3 different cropped versions of image1 (so you can tell the Versions apart). Likewise, create 2 different cropped versions of image2. You should now have 2 Version Sets in each album.
To help keep track of which Version is the pick, press 'T' to turn on image overlays so you can see the Version Title when you pause your mouse over an image.
Click on the number in the top left corner of each Version set to condense them. Notice that the left-most image (the Pick) is the image you se in the condensed Version Set. The same image will appear in Album1 and Album2.
Open the Version Set again, and click on the right-most image, then click the Pick button on the top toolbar (not the Album Pick button; the Pick button). Notice the rightmost image physically moves to the leftmost position. Close the Version Set and notice that this Version is now the one that displays when the Version Set is condensed.
Go into Album2 and condense the Version Set there, too. Notice that the same Version as in Album1 is the Pick. That's because we have not selected an Album Pick yet. The default Pick is the Pick for all albums until you pick an Album Pick for one or more albums.
While in Album2, open the first Version Set and select the right-most image in the Version Set. Now click the Set Album Pick button on the top toolbar. Notice no images move this time (compare that to clicking the Pick button on the toolbar). Instead, a black checkmark in a gray circle appears over the image. This means this Version is the Album Pick. Collapse this Version Set in Album2, and notice that the Album Pick image is what shows in the condensed Version Set. Now go back to Album1, and you'll notice there is no checkmark on any images. Collapse the first Version Set, and you'll see the left-most image (the default Pick) is the image that shows up on the collapsed Version Set. If you wanted to, you could open up the Version Set, select the middle image and click the Set Album Pick button, and the middle image would represent the Version Set in Album1, while the 3rd image would represent the Version Set in Album2. Version Sets can have no Album Picks, in which case the default Pick (the leftmost image) is what is selected for a collapsed Version Set.
OK, let's have some real fun now! Select the first Version Set, then command-click on the second Version Set to select both Version Sets. Now right-click (or control-click) on one of the images and select Stack. Both Version Sets should be combined into a single Stack, but you should still be able to see the light gray border around the two Version Sets within the Stack.
Click outside the Stack to deselect all the Versions. Now click on the left-most Version in the right-most Version Set. Click the Pick button on the top toolbar. Notice the entire right-most Version Set moves to the left to become the (default) Pick! Press command-Z to undo your change.
Now click on the right-most Version in the right-most Version set. Click the Pick button on the top toolbar. Notice that, not only does the entire right-most Version Set move to the left, but also the rightmost Version moves to the left-most position within the Version Set! You're not only telling Aperture to make the second Version Set the Pick for this Stack, you're also telling it that the right-most Version within that Version Set is the Pick for the Stack.
Play around with setting the Album Pick within each Stack and comparing Album1 with Album2 when you collapse the Stack.
This is a fun exercise that can really help you understand the difference (and similarities) between Version Sets, Stacks, (default) Picks, and Album Picks.
Please correct anything that I'm misunderstanding, or feel free to add anything you've found out about these topics.

Answer 1

Aperture's documentation is very good in most
respects (adjustments being the major exception), but
I found it very difficult to put my head around the
whole version set / stacks / picks / album picks
thingy.
While working on some images this morning, it all
became clear.
<...>
I thank you for the clarification on those matters, while I have used stacks quite a bit I have not yet explored in full detail the Version Set concept (though I had a rough idea of what it did). That was a great job of investigation.
To help bring a similar level of understanding to others regarding the Aperture adjustment tools (which as you note are not well documented), I have published an Adjustment Tool Guide (2 MB PDF) that covers all of the Aperture adjustments - you can find it at:
http://www.InsideAperture.com
Just follow the Adjustment Tool Guide links.