Does SC even use these for ground units? afaik it just renders units according to position, from top left to bottom right....
Also is it possible to determine animation level of doodad sprites?
Hmm...I think doodads are all created at a certain hardcoded levels, since they're not units.dat entries. Dunno what level is though, probably related to the tileset they correspond to.
For ground units, yes, the animation level matters when moving in relation to other units/buildings for example. Try giving a unit the lowest anim.level and move it around afew buildings - you'll that it can move about their corners for example.
also if you change a air units animation to below a ground units it will go under it, vise versa for ground and air. You can do the same in a way with ground and ground but you would have to make one able to walk through other units, which you can easily do, but it looks weird.
In STF, the add-on Facilities at one point appeared beneath the Hub. Only by raising their animation level above the Hubs' did they appear on top.