Optical and Mechanical Resolution Optical resolution tells you how many pixels the scan element can see at once across the width of a page. Mechanical resolution tells you how many steps the scan element takes going down the page (or how many the page takes going past the scan element). A spec like "600-by1,200-dpi optical" really means "600-ppi optical by 1,200-ppi mechanical."
N by M Optical Resolution. Not.
Even ignoring the distinction between optical and mechanical resolution, a spec such as "600-by-1,200 optical resolution" is misleading. The scanner can't pass a 600-by-1,200-ppi image to your computer, so the top resolution you'll get without interpolation is 600-by-600 ppi.
High Optical Resolutions Even high optical resolutions usually don't matter. For typical office tasks such as copying, faxing and scanning to PDF files, and even for scanning photos to print at the same size, a 600-ppi scanner is almost always all you need. Higher resolutions are rarely useful unless you're scanning, say, slides or otherwise need to resize the image.