My sprite experiments ended up as a small, spooky age.
This is meant only to add to the Wiki sprite description.
The sprite types are represented by the value (in D'ni) of the least-significant, four bits of the sprite flags.
Sprite 1 keeps its current Y axis aligned to the camera's view of its original Z axis. It is well-behaved near the equator of the original Z axis.
Sprite 2 takes the ray in the original XY plane that is aligned with the camera's view of the original Z axis and aligns that current ray to the camera's view of the original Z axis.
It is well-behaved near the zenith and nadir of the original Z axis.
Sprite 4 aligns the current Y axis with the original Y axis. It is well-behaved near the equator.
Sprite 8 does not impose any alignment. It is well-behaved at all angles. Since arbitrary rotations aren't orthogonal the sprite's orientation is dependent on its rotation history.
The sprite displays in the gallery limit the views to well-behaved geometries. You can see sprite 8 from many angles, but sprite 2 only from angles near the zenith of its original Z axis. The original Z axis points up.
Enjoy the gallery and let me know if any or all of this is nonsense!