You can create stacks to group a set of visually similar photos together, making them easy to manage. Stacks are useful for keeping multiple photos of the same subject or a photo and its virtual copies in one place, and they reduce clutter in the Grid view and the Filmstrip.
For example, you may want to create a stack to group multiple photos of a portrait session taken with the same pose, or for photos taken at an event using your camera’s burst mode or auto-bracket feature. When you take photos this way, you end up with many similar variations of the same photo, but you usually want only the best one to appear in the Grid view or the Filmstrip. Stacking the photos lets you easily access them all in one place instead of having them scattered across rows of thumbnails.
When grouping photos in a stack, the photos are stacked according to their sort order in the Grid view, with the active photo at the top of the stack.
A stack is collapsed when stacked photos are grouped under the thumbnail of the top photo in the Grid view or the Filmstrip. A stack is expanded when all photos in a stack are visible in the Grid view or the Filmstrip.