When you click on a decal, there’s a Zindex and I don’t know what it is. Can you tell me what it do ? Check near to “Transparency” on my following screenshot.
Do like if you don’t see anything, my game isn’t very interesting 
also yes I use the toolbox sometimes… (even almost always)
It’s for when using multiple decals. Example: Decal ZIndex = 1, Decal 2 ZIndex = 2. Decal 2 will be shown in front of Decal 1
The ZIndex to put it simply determines what is shown first.
If you have 2 decals on a part and change one of the decal’s ZIndex to 1 and the other to 0 you’ll see the image in which the ZIndex is set to 1.
How its usually used tho is let’s say you have a few decals to put together in order to make an image, some of the decals you won’t see because they will be covered so you change the ZIndex so you can see them
