When steel is produced at the mill it is manufactured to a target thickness. Variations in the final thickness of the sheet steel are unavoidable. However, thickness is controlled very closely to insure it does not fall below the minimum thickness allowed.
Minimum Allowable Thickness: In the ordering process the customer can specify what minimum thickness is required as measured 1 inch in from the edge of each side of the steel. Galvanized steel (for example) has a 3% crown in the center of the coil so the center will be 3% heavier or thicker than the edges.
Nominal Thickness (mill terminology not to be confused with theoretical nominal billing): Nominal thickness is measured at the centerline of the sheet/coil with plus or minus tolerances. The mill will run the steel at the nominal or targeted thickness centerline allowing for slight variations in thickness either heavier or lighter. When I was buying foreign steel it was normal to request steel to be rolled based on nominal thickness “rolled to the light side” (also use the terminology “aim to” the light side). In other words the target or nominal centerline was set at the decimal required with only minus tolerances and no plus thickness tolerance allowed.