A gallery's cover image is a single image from that gallery, selected according to a particular criterion. For example, the cover for each gallery might be the most recent image in that gallery. If a gallery has no descendant galleries, this is a simple procedure. However, what to do when the gallery itself contains galleries is a more complicated matter. There are three ways to handle descendant galleries when getting a cover image for a gallery.

Plain
  Do not go into any descendant galleries: only consider images in the gallery itself. If the gallery itself has no images, then there will be no cover node.
Flat
  Consider the gallery itself and its descendants to a given depth as one single pool of images from which to select the cover.
Recursion
  Consider first the gallery itself, and if it is empty, consider each child gallery one at a time (in weight order). The recusion depth sets how far to go into a child gallery's own children before giving up and moving to the next child gallery. Be warned that this is potentially a time-consuming procedure!

These methods are illustrated in this example. Consider the following tree of galleries: the numbers are the term IDs that make the galleries and the letters are the titles of the image nodes.

      - 1: empty
        - 2: empty
          - 6: nodes G, H
        - 3: nodes M, N
        - 4: nodes D, E, F
        - 5: nodes X, Y, Z
          - 7: nodes A, B

Suppose our sort criterion for the cover is the image node title in alphabetical order. What cover image will gallery 1 have?

Plain
  Term 1 has no nodes: there is no cover image.
Flat depth 0; Recursion depth 0
  Depth 0 means we don't go into child galleries: so this is the same as plain and there is no cover image.
Flat depth 1
  We consider all nodes in the galleries 1, 2, 3, 4, 5. The first node by name is D.
Flat depth 2
  We consider all nodes in the galleries 1, 2, 3, 4, 5, 6, 7. The first node by name is A.
Recursion depth 1
  We find the gallery itself empty, so look in gallery 2. That's empty too, so we look in gallery 3. Here we find nodes, and M is the first by name: so the cover is M.
Recursion depth 2
  We find the gallery itself empty, so look in gallery 2. That's empty too, and so we go one level deeper to gallery 6. Here we find nodes, and G is the first by name: so the cover is G. Node A is a better fit at this depth, but we consider child galleries one by one, by weight, and we found something in gallery 6.