I have 3 questions about subbiomes and layers in biomes.xml

[xxx73]

Hi

I'm going to add several new resources to the biomes, and I like to make new subbiomes. To do that I'm trying to understand a few things.
 

1. Is there a limit to how many subbiomes one biome can have?

 

2. Each biomes is setup like this in biomes.xml;
First weather and then subbiomes, most if this I understand.
But at the bottom below the subbiomes come somethings called layers and decorations that is not connected to any subbiome (using snow as example).
In a subbiome I understand layers and decorations, but this is not connected to a subbiome so what is this?

 

        <!-- Block layers: Snow -->
        <layers>
            <layer depth="1" blockname="terrSnow"/> <layer depth="3" blockname="terrDirt"/>
            <layer depth="*" blockname="terrStone">
                <resource blockname="terrOreIron" prob="0.6500" rwgGenerationType="all"/>
                <resource blockname="terrGravel" prob="0.7830" rwgGenerationType="all"/>
            </layer>
            <layer depth="3" blockname="terrBedrock"/>
        </layers>

        <!-- Decorations Main: Snow -->
        <decorations>
            <decoration type="prefab" name="deco_iron_vein" checkresource="-7" onslopes="true" prob=".00657" rotatemax="3"/>
            <decoration type="block" blockname="rockResource" prob=".00033" rotatemax="3"/>
            <decoration type="block" blockname="rockResource02" prob=".00033" rotatemax="3"/>
            <decoration type="prefab" name="rock_form02" prob=".0005" rotatemax="7"/>
            <decoration type="prefab" name="rock_form01" prob=".0005" rotatemax="7"/>
            <decoration type="block" blockname="cntForestRandomLootHelper" prob="0.00025"/>
            <decoration type="block" blockname="rock05" prob=".005"/>
            <decoration type="block" blockname="rock06" prob=".005"/>
            <decoration type="block" blockname="rock07" prob=".005"/>
            <decoration type="block" blockname="cntBirdnest" prob="0.001"/>
            <decoration type="block" blockname="treeWinterEverGreen" prob="0.003" rotatemax="7"/>
            <decoration type="block" blockname="treeWinterPine13m" prob="0.003" rotatemax="7"/>
            <decoration type="block" blockname="treeWinterPine19m" prob="0.003" rotatemax="7"/>
            <decoration type="block" blockname="treeWinterPine28m" prob="0.003" rotatemax="7"/>
            <decoration type="block" blockname="treeSnowyDeadBush" prob="0.005" rotatemax="7"/>
            <decoration type="block" blockname="treeSnowyGrassDiagonal" prob=".1"/>
            <decoration type="block" blockname="treeStump" prob=".0002" rotatemax="7"/>
        </decorations>

What is this and how does it work?
Is this some kind of "default biome" that get created if no other subbiome gets chosen?
If that's the case, how often does this "default biome" get created?

If I reduce probability of all subbiomes, will I get more of this "deafult biome"?

3. Some of the resources I'm adding is suppose to be rare, but I like to give them separate subbiomes. Only problem is that if adding a rare resource with gravel it all will collapse. To avoid cave collapse, can I add stones in that subiome?
 

[xxx73]

Here is an update so far on my testing.
1. 8 different subbiomes seem to work fine.

 

2. As I expected the layers and decorations at the bottom of each biome seem to behave like filler when no subbiome is selected.
Reducing subbiomes probability cause more of this bottom layers and decorations to spawn.

3. It work well to add stones to a vein to have other fill materials than gravel.

 

[xxx73]

I have done a lot of testing with subbiome probability and there is something that seems weird.
If I reduce probability of all subbiomes there is less subiomes spawned, if I increase all the there is more.

 

But the balance between the subiomes seem to stay the same even if i set probability in some very low, or increase some others, the composition of the different subbiomes seem to stay the same.

Is the composition/balance between the different subbiomes hardcoded?

[convergent]

The probability is really the size. There is a granular map generated of subbiomes- if you make the same map with parameters all the same except biome prob, the sub will be in exactly the same spot, just larger or smaller. Of course, the more sub types you have, the less likely a sub point is to be a particular type. If you have 2 different probs there will be the same number, just less area.