As an adjunct to Smoochas guide on this topic. MILLS is a unit of measurement used often in the electronics industry. Printed circuit board track widths are often expressed in MILLS as distinct from millimeters. For example 10 MILL = 0.254mm. Therefore a 100MILLS plated bar would have a plate thickness of 10x0.254mm=2.54mm. Presumably this would be on both sides meaning a plated thickness on EACH side of the bar of 1.27 millimeters. The following link may be helpful...you can enter the thickness in the mills box to obtain the value in millimeters.
Just google PCB Track Width Calculator to find any number of calculators which will demonstrate this.
Hope this helps.