Optical resolution and minimum pixel size
These two specifications are related. A higher resolution is better (pixel size is smaller). The optical resolution is the highest resolution at which the images are scanned without interpolation. Maximum resolution is the highest resolution a scanner can produce with interpolation. Interpolated resolution is calculated after the scanner adds theoretical pixels to an image, it averages the colors of neighboring pixels to create additional pixels; something which may help to enhance details already seen by the scanner. The goal for a good scanner is to achieve the best resolvability to distinguish fine details (like tree-ring transitions). There are large differences among scanners and sometimes their theoretical specifications do not match their actual performances. This is because it is not only the sampling frequency (dpi) that is important but also the quality of the optical, electronic and mechanical components. Even if you don't plan to scan at 2000 dpi, a 2000 dpi scanner usually produce better scans at 600 dpi than a 600 dpi scanner.