Rough Sawn, RS, means that the timber will be rough on all four sides and measure to the full measurement of the timber. For example, an 8x8x8 will measure approximately 8 inches by 8 inches by 8 feet for a true 8x8x8.
Smooth Sawn, S4S (smooth on four sides), is a timber that has been milled on four sides to provide a smooth surface for the end consumer. Plus, an 8x8x8 timber would be a nominal size of 7 ½ inches by 7 ½ inches compared to the Rough Sawn.