Scan gap, line gap, scan interval, and line spacing are all terms for the same thing. It is the amount that the laser moves between lines when doing a scan engraving. Scan engraving is when the laser goes back and forth, line by line, engraving an image. If your scan gap is too big you will notice blocky edges (aliasing) and possibly white space between lines. Lowering the scan gap will improve your resolution and quality but will take longer and may result in overlapping lines if the gap becomes too small.