Confused about the Dimension Grid

[LunarWarg]

In the default vanilla Dimension Grid, why are there surface biome slices located below the cave slices in the depth field? And how do the valley, low slice, etc. interact with weirdness? For example, how does the valley slice at weirdness [−0.050, 0.050] select the low grooves throughout the world?

[jacobsjo]

why are there surface biome slices located below the cave slices in the depth field

This is a vanilla design decision. Cave biomes don't reach all the way down to bedrock, so if you go down from cave biomes you get the surface biomes again.

how do the valley, low slice, etc. interact with weirdness? For example, how does the valley slice at weirdness [−0.050, 0.050] select the low grooves throughout the world?

I'm not sure what you mean. Are you referring to the triangles in the grids? This is determined by the "A"/"B" toggle at the top of the dimension grid. If its set to "A" then it uses the top triangle for that slice, if its "B" it used the bottom triangle. Since the x-axis of the Dimension grid is weirdness, this allows you to determine which triangle is used per weirdness.

In case you are confused about the valley slice specifically: Yes that only uses the bottom triangle even in the [-0.050, 0] range. That is what vanilla does. You could of course change that by splitting that column into 2 and set it to use the top triangle in the left one.