units

Basically u it is 3 ounce not 30z

normally where this unit get used

for which material?

"Ounces" are units in the Traditional British, or inch-based system* of measurement. It is still used in the USA, Myanmar, and I believe Liberia. Your best choice is to ignore it entirely. Ounces may refer to weight [not mass, usually] or to force, or to fluid volume, among other possibilities. As a measure of weight, an ounce is 1/16th of a pound (a pound at 1g is equal to 453.5924 grams). There are also several KINDS of ounces and pounds, not equal to each other, used for weighing some intrinsically valuable materials. Thus, to avoid splitting headaches, stick to the metric system.*

*"System" is a misnomer with regard to Traditional British units, as there is nothing systematic about it! It is a hodgepodge, a compilation, a compendium, an aggregate, but certainly NOT a system.

z = ounce

British Standard, like pounds.