ZIndex determines the order in which decals on the same
Decal/Face|Face of a
BasePart are rendered. Decals are rendered in ascending priority order, where lower values are rendered first. Therefore, a decal with a higher ZIndex renders later (and on top of) other Decals with lower ZIndex.
The range of valid values is -MAX_INT to MAX_INT, inclusive (2,147,483,647 or (2^31 - 1)). If you are unsure if you will need to layer an decal between two already-existing decals in the future, it can be a good idea to use multiples of 100, i.e. 0, 100, 200. This ensures a large gap of ZIndex values you can use for elements rendered in-between other elements.
GuiObject/ZIndex, a property which behaves similarly, but for GUI elements