I can help with depth. Assume I have a group of mini trees. There are 10 mini trees in the group (each tree is a group of legs). each tree has 10 legs (each leg is a group of pixels), each leg has 10 pixels.

I apply a chase to the top level element called "mini-Trees"

Depth determines: Am I chasing the individual trees (level 0), chasing the legs on the trees (level 1) or chasing the pixels on the legs (level 3).

Those level numbers may be off by one but that is the basic idea.