items.xml: armor mask parameters?

[VicWint]

In items.xml, when specifying a piece of armor, you can have UMA properties like 

 

<property name="Mask0" value="Head" param1="inner"/>

 

I tried making an armor piece which uses the same mesh as a clothing piece and left out this mask stuff and got some z fighting when wearing both the armor and clothing.

 

And, since my experiments are slow (I have to start a new game each time I change a config file), I thought I should ask about this, to see if anyone can throw me some hints.

 

I have seen four different param values for these masks:

 

"inner" (common)

"outer" (common)

"middle" (less common)

"base" (rare)

 

What do these mask values mean?