Hi
Do you mean what it was manufactured for or what it's used for in terrariums?? When used in light fixtures they call it a "difuser"
gary

It is a light diffuser and you will find it in the light section of a hardware store. It is used to creat a false bottom so that if you have a water feature then the water has somewhere to drain to without soaking your substrate and you have somewhere for the pump to sit, etc. You can accomplish the same thing sort of by just using something like gravel for a drainage layer under your substrate, but this really cuts down on the weight of a tank and that is good if you ever need to move it.