Stacks let you group files together under a single thumbnail. You can stack any type of file. For example, use stacks to organize image sequences, which often comprise many image files.
Commands that apply to a single file also apply to stacks. For example, you can label a stack just as you would a single file. Commands you apply to expanded stacks apply to all files in the stack. Commands you apply to collapsed stacks apply only to the top file in the stack (if you’ve selected only the top file in the stack) or to all files in the stack (if you’ve selected all files in the stack by clicking the stack border).
The default sort order in a stack is based on the sort order for the folder that contains the stack.
In stacks that contain 10 or more images, you can preview (scrub) the images, specify a frame rate, and enable onion skinning, which allows you to see preceding and succeeding frames as semitransparent overlays on the current frame.