Theres a similar section in the help files within the app covering this but its probably a good thing to reiterate it as its so vital. Incidentally, all the help is in ViewingDale format as is all of the artwork used to make the laminated card, insert, and box cover.
As you recall from the intro what were after is a mechanism to have the whole world at your fingertips and we would like to be able to reuse those tree icons to make copses, woods and forests. What were building here is a hierarchy.
The main view looks at a map. Actually your looking at one layer in an infinite set. This layer is called an icon. An icon is just a bunch of a small set of parameters one of which is a reference to an image. The image is whats shown on the screen and its sized to extents which are parameters in the icon. So you can have two different icons with the same image. I.e. you can have a small chair and a giant chair. Two different icons using same image. Most of the time you have one icon per image because many of the icons are so unique but things like arrows, lines, circles and so on have many icons for the one image. The other thing that an icon has is a list of child icons attached to this one. I.e. for an icon of a village it might have a list of child icons, one for each hut in it. Those child icons are listed with their position, scale and rotations for each one.
So we can have a Forest which has 3 child icons each of type Wood and those have positions and rotations so that they are next to each other making up a little group.
Now if the Wood icon happens to have 6 children called Copse arranged in a little group also then when you view the Forest then it will show its icons image, 3 Wood images and 18 Copse images. And it goes on and on like that. If you add another Wood to the forest then the forest icon gains one more child but 6 more Copse images are also shown.
So the ViewingDale app has controls to edit the icon size, add and remove child icons and to position, rotate and scale the child icons on the main one.