What's the copper weight?

The unit of copper foil thickness is ounces per square foot(oz/ft­2), the unit also simply called ounce. Common thicknesses are 1 oz/ft2 (300 g/m2), 2 oz/ft2 (600 g/m2), and 3 oz/ft2 (900 g/m2). These work out to thicknesses of 35 µm(1oz),70 µm, and 105 µm, respectively. 

These parameters are used for top/bottom circuit layers, as for the inner layer, the copper weight is all of 0.5oz. Increase the copper weight can get a better electrical properties, however, it may be difficult to etch the board.