I'm struggling to understand exactly what U and V are in the Formula Effector. The documentation states:

```
d(uvxyzt)
This is where you enter the formula for all effects apart from Manual. u and v are parameters that run from 0 to 1 along the horizontal and vertical axes respectively.
```

But this definition doesn't make it clear how that's different than the y or x axis. If I apply a formula effector to a sphere, here are some results I get:

When I just use "U" in the formula:

When I just use "V" in the formula:

I don't see the relationship between "the horizontal and vertical axes" in this case.

Does somebody have a more precise definition of what u and v represent, specifically in "spherical" mode? What do they range from (is it 0 to 1)?